Doc Polishing for the AMQP Support

Documentation for configuring an external listener container in
the inbound endpoints.
INT-3164: Fix Scripting Refresh

Previously, there was no logic to check, if a JSR223 script
was modified should be refreshed from the external resource.

* Fix `RefreshableResourceScriptSource` to take care of refreshing
on call `isModified()`
* Add `scriptSource.isModified()` before getting script text
* Introduce `ScriptSourceFactoryBean` to avoid I/O operations, when
there is need to refresh script resource - `refreshDelay < 0`

JIRA: https://jira.springsource.org/browse/INT-3164

INT-3164: change the refresh mutation logic

Remove factory bean - no longer needed.
INT-3166: Improve Groovy processor performance

Previously, there was a `synchronized` block in the `GroovyScriptExecutingMessageProcessor`
to achieve thread-safety around script variables.
In addition parsing and executing logic was delegated to `GroovyScriptFactory`, who, in turn,
has its own `synchronized` around script parsing.
It's not good in messaging architecture.

* Introduce `GroovyScriptExecutor implements ScriptExecutor`
to encapsulate Groovy parsing and executing logic
* synchronize via `ReentrantLock` only when script resource `isModified()`

JIRA: https://jira.springsource.org/browse/INT-3166
INT-3166: Fixes and Polishing
INT-3166: add synchronized double check