INT-3146 TCP SO Timeout with Caching Client CF

https://jira.springsource.org/browse/INT-3146

Incompatibility of using socket timeouts with a caching client
connection factory.

When a socket option timeout (soTimeout) is set on a TCP connection
and the timeout occurs, the socket is closed.

When a connection is intercepted, the close is performed through
the interceptor (for example to allow a closing handshake before the
physical close).

However, the caching client connection factory is implemented
using an interceptor, which returns the underlying connection to
the cache pool (for reuse). In the case of a TcpNetConnection,
the reader thread has terminated meaning that, the next time the connection
is used, no reply will ever be received.

The work-around (when using a gateway) is to use the 'remote-timeout'
attribute instead of relying on the soTimeout. There is no
work around when using collaborating channel adapters with a Net
connection. Using NIO works because there is no reader thread
in that case, but the socket is never closed on a timeout.

Always physically close the connection whenever an exception occurs
even if the close was delegated to an interceptor.

Add test cases for Net and NIO implementations.
INT-3146 Fix Test Timing Issue

Wait for server connection factory to start listening for new tests.
INT-3027: Headers Enrichment for <enricher>

* Add `<header>` sub-element to `<enricher>`
* Refactoring of `EnricherParser`
* Add Headers Enrichment logic to `ContentEnricher`
* Add tests
* What's new and Content Enricher's `<header>` note

JIRA: https://jira.springsource.org/browse/INT-3027

INT-3027: Addressing PR comments

INT-3027: add NPE asserts to map properties

INT-3027: add overwrite & type to enricher header

Move `HeaderValueMessageProcessor` hierarchy to 'transformer.support' package

INT-3027: Make header's overwrite=true by default

Doc Polishing
INT-3146 Fix Another Test Timing Issue

Increase test timer for soTimeout detection.
INT-3133: Add <spel-property-accessors> Support

* Add <spel-property-accessors> to configure a list of beans
that implement `org.springframework.expression.PropertyAccessor`
* Add `SpelPropertyAccessorRegistrar` to manage a list of PropertyAccessors
for Integration infrastructure
* Refactoring for `SpelFunctionRegistrar` and `IntegrationEvaluationContextFactoryBean`
to fix the issue when there is no `SpelFunctionRegistrar`(`SpelPropertyAccessorRegistrar`)
in the child AC, but there is one in the parent.
Previously the inheritance did't work for that reason.
* Now parent/child logic moved to `IntegrationEvaluationContextFactoryBean`
* Add documentation

JIRA: https://jira.springsource.org/browse/INT-3133

INT-3133: Rebasing, polishing and documentation

INT-3133: PropertyAccessors override support

* `@Ignore` `SOLingerTests#finReceivedNioLinger()` test

INT-3133: Change JavaDoc link to 3.0.0.RC1

Polishing

- Remove duplicate check - last bean def wins
- Doc Polishing
INT-3133 Doc Polishing