JndiPropertySource defensively skips invalid JNDI lookup for property name with colon in resource-ref mode

Issue: SPR-14518

With the current state machine
- the implementation can hang after the last element when executing
on Jetty.
- in some cases there will be no flush after the last
Publisher<DataBuffer>.

AbstractResponseBodyProcessor.onError and
AbstractResponseBodyFlushProcessor.onError will be invoked when:
- The Publisher wants to signal with onError that there are failures.
Once onError is invoked the Subscription should be considered canceled.
- The internal implementation wants to signal with onError that there
are failures. In this use case the implementation should invoke
Subscription.cancel()

This commit changes writeWithInternal(Publisher<DataBuffer> body).
It is implemented as writeAndFlushWith(Mono.just(body)).

Issue: SPR-14517

Expose DefaultSubscriptionRegistry's cache limit through SimpleBrokerMessageHandler and MessageBrokerRegistry

Issue: SPR-14516

Issue: SPR-13344

Issue: SPR-14511

Issue: SPR-13188

DataSourceUtils moved to main core.io.buffer package.
Consistently named Jackson2JsonDecoder/Encoder and Jaxb2XmlDecoder/Encoder.
Plenty of related polishing.

Drop JpaSystemException(PersistenceException) constructor to avoid ambiguity in HibernateJpaVendorAdapter

Hibernate 5.2's HibernateException extends PersistenceException, leading to a different constructor compiled into the bytecode, breaking compatibility with Hibernate 5.0 at runtime.

Issue: SPR-13827

Issue: SPR-14497

The CORS section is now ahead of
"Integrating with other web frameworks" and "WebSocket Support".

Issue: SPR-13344

Issue: SPR-14430

Issue: SPR-14486

This commit also drops the now-unused CUSTOM_COMPILATION test group and the outdated JavaVersion class.

Issue: SPR-10423

* pr/1112:
  Polish contribution
  Polish Javadoc

Closes gh-1112

See gh-1112

Issue: SPR-14509

Issue: SPR-13188

Issue: SPR-11569

Issue: SPR-13575