Closes gh-4760

Google App Engine probits the creation of new threads. This leads to a
failure in BackgroundPreinitializer when the single thread executor
attempts to create its single thread.

This commit enhances the existing fail safety of
BackgroundPreinitializer by catching any exceptions thrown while
creating the executor and submitting the tasks to it. Any initialisation
that has not performed in the background will be performed in the
foreground instead.

Closes gh-4662

* pr/4766:
  Use canonical paths for Undertow document root

Update `UndertowEmbeddedServletContainerFactory` so that the canonical
path is used when setting up the document root. Prior to this commit
Windows machines with `java.io.tmpdir` set to a tilde-compressed path
would cause problems.

Fixes gh-4765
Closes gh-4766

Previously, the default RestTemplate that is used OpenTsdbGaugeWriter
was not used with its default configuration. Notably this meant that
it would have infinite connect and read timeouts. This is problematic
as it can cause metric writing to hang and block the scheduler for
performing any other tasks.

This commit updates OpenTsdbGaugeWriter to use a default connect
timeout of 10 seconds and a default read timeout of 30 seconds. A
constructor has been added to ease the configuration of these
timeouts. The existing option of providing your own RestTemplate
(via setRestTemplate) remains.

Closes gh-4698

Closes gh-4731

- Consistent error handling applied to BasicJsonParser,
  GsonJsonParser, JsonSimpleJsonParser and YamlJsonParser
- Add tests in AbstractJsonParserTests to verify consistency
- Rename tests for JsonSimpleJsonParser to match the name of the
  class under test.

Closes gh-4690

Previously, when ignoreUnknownFields was false and property names were
being filtered based on whether or not they begin with the target name,
relaxed variants of the target name were not considered. This resulted
in different delimiters resulting in a non-match. For example, the
property ENV_FOO_NAME would be filtered out when the target name
was env.foo.

This commit updates PropertiesConfigurationFactory to pass all of the
relaxed variants for the target name to the matcher. For the example
above one of those variants will be env_foo which matches ENV_FOO_NAME
due to the matching delimiter.

PropertiesConfigurationFactory was already creating a RelaxedNames
instance for the target name. The code has been reworked a little to
allow these relaxed names to be reused, thereby avoiding the cost of
computing all of the relaxed variants of the target name a second time.

Closes gh-4775

Update ConfigurationWarningsApplicationContextInitializer to also log
warnings if the user is scanning `org` or `org.springframework`.

Fixes gh-4777

Closes gh-4776

See gh-4763

Prevent potential SpEL injection attacks by ensuring that whitelabel
error view SpEL placeholders are not recursively resolved.

Fixes gh-4763

* pr/4753:
  Polish contribution
  Add constants for banner location

Closes gh-4753

Closes gh-4665

Previously, ActiveMQ's pooled connection factory was not closed as
part of the application context being closed. This would leave
non-daemon threads running which could cause shutdown to hang unless
the JVM itself was shutting down (in which case a shutdown hook would
stop the pool).

This commit configures each pooled connection factory bean with a
custom destroy method so that the pool is stopped as part of the
application context being closed. To allow the destroy method to only
be declared when the connection factory is pooled, the bean method
has been split into two; one for pooled and one for non-pooled. This
is a partial backport of the changes made in bedf2edf.

Closes gh-4748

Closes gh-4756

* pr/4770:
  Polish contribution
  Rename method to clarify which connector is being created

Closes gh-4770

See gh-4770

See gh-4769

Ensure that gzip compression is applied when the `Content-Length` header
is not specified. Prior to this commit Tomcat and Jetty would compress a
response that didn't contain the header, but Undertow would not.

Fixes gh-4769

* pr/4504:
  Add support for server.server-header property

Add a `server.server-header` property which can be used to override the
`server` header usually sent back automatically by Tomcat/Jetty or
Undertow.

See https://www.owasp.org/index.php/Securing_tomcat for background.

Fixes gh-4461
Closes gh-4504

* pr/4768:
  Polish docs

Closes gh-4768

* pr/4625:
  Move publisher injection to abstract listeners

Push up publisher injection from AuthenticationAuditListener and
AuthorizationAuditListener to the abstract superclasses.

Closes gh-4625

Extract exit code logic from SpringApplication to a new
`ExitCodeGenerators` class so that it can be reused.

Fixes gh-4757

Update `removeLineBreaks` to also work with `null`.

See gh-4703

* pr/4672:
  Protect against Math.abs() with Long.MIN_VALUE

Update RandomValuePropertySource to protect against the random source
returning Long.MIN_VALUE. In such cases the sign bit can't be unset and
prior to this commit the random value stayed negative.

Closes gh-4672

* pr/4674:
  Add AbstractHealthAggregator.aggregateDetails

Extract aggregate details logic to a protected method that can be
overridden if required.

Closes gh-4674

* pr/4688:
  Prefer valueOf() to create Number values

Update Long/Integer constructor calls with `valueOf` which can make use
of global caches.

Closes gh-4688

* pr/4703:
  Cleanup description new-lines for launch script