INT-3166: Improve Groovy Processor Performance

Previously, there was a `synchronized` block in the `GroovyScriptExecutingMessageProcessor`
to achieve thread-safety around script variables.
In addition, the parsing and executing logic was delegated to `GroovyScriptFactory`, who, in turn,
has its own `synchronized` around script parsing.
This causes a bottleneck in a 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

INT-3166: Remove `GroovyScriptExecutor`

Polishing tests and logic around exceptions

Fix up some white space issues in Groovy*Tests