INT-3064 Change MessageHeaders.Id Strategy

Use `com.eaio.uuid` by default and document how to replace
the default strategy.

Provide JDK and a simple incrementing incrementing implementation.

Add code to detect multiple contexts in the same classloaded using
the same strategy.

Add code to detect multiple IdGenerator beans and emit a WARN
instead of DEBUG, which is the case for no beans.
INT-3064 removed 2 obsolete tests
INT-3082 (S)FTP Remove Deprecated 'cache-sessions'

The 'cache-sessions' attribute was deprecated in 2.1 in favor
of using a configured CachingConnectionFactory to provide more
flexibility, such as setting the cache size.

Remove the attribute from the schemas.

Update tests.

Add docs.

Update Migration Guide on the project Wiki.