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
f0c4bdb756