Closes gh-4018

Update `ServerProperties` to automatically enable `use-forward-headers`
when running on a cloud platform.

A new `CloudPlatform` enum has been introduced that detects Heroku and
Cloud Foundry.

See gh-4018

Add a new `server.use-forward-headers` property which can be used to
switch on X-Forwarded-For header support in all supported embedded
servlet containers.

This commit reverts the decision to enable `RemoteIpValve` with Tomcat
by default (gh-3782) and requires that either `user-forward-headers` is
set to true or that `server.tomcat.protocol-header` or
`server.tomcat.remote-ip-header` are set.

See gh-4018
See gh-3782

Add a `useForwardHeaders` property to allow embedded Undertow
containers to respect X-Forwarded-For headers.

Fixes gh-3881

Add a `useForwardHeaders` property to allow embedded Jetty containers
to respect X-Forwarded-For headers.

Fixes gh-3802

Update Abstract & Tomcat EmbeddedServletContainerFactoryTests to check
that X-Forwarded-For headers work as expected.

See gh-4018

Update ManagementWebSecurityAutoConfiguration to reinstate lazy creation
of EndpointHandlerMapping from the EndpointPathRequestMatcher.

Fixes a regression introduced in eb298478 and picked up my one of the
sample integration tests.

Fixes gh-4059

See gh-4068

After the ApplicationEnvironmentPrepared we now check that the
webEnvironment flag and/or the Environment hasn't changed, in case
the user wanted to switch the context from a web to non-web in a
listener.

Fixes gh-2716

In order to have more control on the structure of the configuration, we
do not bind to `RepositoryRestConfiguration` directly anymore. This
commit introduces `RepositoryProperties` instead.

See gh-3854
Closes gh-4073

Takes into account the fact that the new /actuator endpoint sometimes
loses its path (it is "" relative to a non-empty management context path).

Fixes gh-4059

* pr/4069:
  Polish

Closes gh-4069

Update devtools to detect JRebel and disable application restarts. Other
features (such as LiveReload) can still be used.

Fixes gh-3095

For logback we also support logging.pattern.level as a synonym.

Fixes gh-4062

Closes gh-4049

Closes gh-3949

Previously, a `NoClassDefFoundError` could be thrown if EhCache or Guava
is on the classpath without `spring-context-support` as the respective
cache statistics provider both needs the implementation and the spring
abstraction implementation.

The `@ConditionalOnClass` definition has been updated to properly guard
those beans.

Closes gh-3960

Previously, if a file or non-existent directory was added to
FileSystemWatcher, it would fail with the message “Folder must not be a
file”. While it suggests that the folder needs to be a directory, it
doesn’t make it clear that it also needs to exist. It also doesn’t
tell the user which folder caused the problem.

This commit updates the message to make it clear that the folder must
exist and must be a directory, and the include the name of the
problematic folder in the error message.

Closes gh-3918

 - Add @author tag
 - Remove unnecessary final modifiers
 - Avoid writing to volatile field when new gauge is used

Closes gh-3977

The previous implementation would remove and add a new Gauge each time
a metric was written. After this change the Gauge is registered once
and the value is updated on subsequent calls.

See gh-3997

This commit reverts the changes made for gh-4026. Those changes updated
each LoggingSystem to close/stop the underlying logging system as part
of the clean up processing. Unfortunately, this approach doesn’t work
in an environment where their are multiple application contexts and
some have a shorter lifecycle than the “main” application context. In
such an environment, closing an application context with a shorter
lifecycle prior to the main application context being closed will
close/stop the main application context’s logging system as, rather than
being scoped to an application context, a logging system is shared
across multiple application contexts. (The exact details of how widely
shared the logging system is varies between logging systems and, in the
case of Logback and Log4J2, also depends on which ContextSelector
implementation is being used.

Closes gh-2516

* pr/4054:
  Fix typo

Closes gh-4054

Update RelaxedDataBinder so that both `.` and `_` are considered in
getPropertyValuesForNamePrefix(...).

With Spring Boot 1.2.5 binding environment variables of the form
`FOO_BAR_BAZ` to `@ConfigurationProperties(prefix="foo-bar")` objects
worked thanks to a happy accident. When `PropertySourcesPropertyValues`
processed a non enumerable `PropertySource` it called the resolver
with a property name `FOO_BAR.BAZ`. A `SystemEnvironmentPropertySource`
will replace `.` with `_` and hence find a value.

Commit 1abd0879 updated non enumerable processing such that the resolver
was never called.

Replicating the problem is quite involved as you need to ensure that you
have both a SystemEnvironmentPropertySource and a non-enumerable
property source (e.g. RandomPropertySource). A test has been added to
PropertiesConfigurationFactoryTests which passes on 1.2.5, fails on
1.2.6 and passes again following this commit.

Fixes gh-4045

This commit updates a number of tests in spring-boot to ensure that
they close the application contexts that they create. SimpleMainTests
still create a number of contexts that are not closed as there’s no
(easy) way to get hold of the context when testing the main method.

See gh-4053

See gh-4026