Closes gh-6333

Closes gh-6339

Previously, one SpringApplicationAdminMXBeanRegistrar was created
per context. When there was more then one context this would result
in a javax.management.InstanceAlreadyExistsException being thrown
as an attempt was made to register the MBean more than once.

This commit updates SpringApplicationAdminJmxAutoConfiguration so
that the registrar is only created when there's no such existing bean
in the context hierarchy.

Closes gh-6378

- Call get rather than containsKey then get
- Only call putIfAbsent after get has returned null to avoid unnecessary
  object creation

Closes gh-6382

Closes gh-6377

* gh-6379:
  Improves metrics performance by not guarding map.get

ConcurrentHashMap implements `containsKey` with `get`. By removing a
redundant call to `containsKey`, we guarantee better performance in our
counter services.

The geek inside measured this with JMH, and found under 4 threads of
contention, throughput on this check was 40% higher in success case.

Benchmark                                  Mode  Cnt     Score     Error   Units
TestBenchmarks.containsKeyAndGet_success  thrpt   30   432.389 ±  20.616  ops/us
TestBenchmarks.get_success                thrpt   30   606.789 ±  10.848  ops/us

Closes gh-6379

* gh-6381:
  Upgrade to Thymeleaf 2.1.5.RELEASE

Closes gh-6381

Closes gh-4393

Previously, if there was a hierarchy of SpringApplications, the admin
MBean would report that the application was ready as soon as any
application in the hierarchy was ready. This could lead to a client
trying to query a property in the environment before it's available.

This commit updates the MBean registrar to that the MBean only reports
that the application is ready when the context that contains the
registrar has refreshed and fired its ApplicationReadyEvent.

Closes gh-6362

* pr/6374:
  Polish

Closes gh-6374

Update SpringBootTestContextBootstrapper to consider the
`@WebAppConfiguration` annotation and use sensible resourceBasePath
defaults.

Fixes gh-6371

Use a custom GeneratorStrategy to fix the package tangles that jOOQ
creates by default.

Fixes gh-6372

Relocate `SpringBootPlugin` to fix a package tangle.

Fixes gh-6355

Closes gh-6360

Closes gh-6369

Closes gh-6008

Closes gh-6357

Previously, the shutdown endpoint would spawn a new thread to perform
the shutdown but did not explicitly configure its thread context
class loader (TCCL). This mean that the new thread would use the
request thread's TCCL as its TCCL. This meant that a different TCCL
would be used compared to a shutdown triggered by the shutdown hook
and also caused problems with Tomcat's thread leak detection logic.

This commit updates the shutdown endpoint to explicitly configure the
TCCL of the shutdown thread to be the ClassLoader that loaded the
endpoint's class.

Closes gh-6361

Closes gh-4892

Previously, the shutdown endpoint would spawn a new thread to perform
the shutdown but did not explicitly configure its thread context
class loader (TCCL). This mean that the new thread would use the
request thread's TCCL as its TCCL. This meant that a different TCCL
would be used compared to a shutdown triggered by the shutdown hook
and also caused problems with Tomcat's thread leak detection logic.

This commit updates the shutdown endpoint to explicitly configure the
TCCL of the shutdown thread to be the ClassLoader that loaded the
endpoint's class.

Closes gh-6361

Closes gh-6358

* pr/6365:
  Upgrade to Spring Integration Java DSL 1.1.3.RELEASE

Closes gh-6365

Closes gh-6344

* pr/6350:
  Upgrade to Spring Security 4.1.1.RELEASE

Closes gh-6350

Closes gh-6340