INT-1362-2: Reset RequestContextHolder in Tests

JIRA: https://jira.springsource.org/browse/INT-1362
INT-3181: Improve JdbcChannelMS usingIdCache doc

JIRA: https://jira.springsource.org/browse/INT-3181

INT-3181: Fix JdbcChannelMS#pollMessageFromGroup

* Add `doRemoveMessageFromGroup` and check its result.
If it is `false` then just return `null`, not polled `Message`
* Add `AbstractTxTimeoutMessageStoreTests#testInt3181ConcurrentPolling` test
and implement it for some DBs
* Make a note regarding MVCC as important in the Doc

Minor Polishing
INT-3185 Fix Direct Handlers When Exposed as MBean

When determining candidates for direct invocation, we need
to use the target of any existing proxy (such as JMX Metrics).

Copy ServiceActivatorDefaultFrameworkMethodTests to the JMX project
and adjust to work with JMX Proxies.

INT-3185: fix for AbstractSMHFBean#checkReuse

JIRA: https://jira.springsource.org/browse/INT-3185