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