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
INT-2629 Extract Gateway Argument Mapping Strategy

https://jira.springsource.org/browse/INT-2629

Strategy for mapping gateway methods to map method arguments
to a message.

Default strategy works as today, but pluggable using attribute
'mapper' on <gateway/>.

A custom mapper should implement InboundMessageMapper<MethodArgsHolder>

where the MethodArgsHolder contains the Method object and the argument
values.

When using a custom mapper, the mapper is entirely responsible for
creating the Message - therefore 'payload-expression' attributes
and <header/> elements are not allowed.
INT-2629 Polishing - Javadocs
INT-2629 Polishing

- Introduce MethodArgsMessageMapper - higher level API for custom mapper.