MODE-1818 Repository configuration can now dictate garbage collection options.

The garbage collection options can now be specified in the repository
configuration. Each repository registers its own garbage collection
processes with a named thread pool, and these processes are stopped
when the repository is shut down.
MODE-1818 Added garbage collection properties to AS7 schema and subsystem

Added the garbage collection configuration attributes to ModeShape's
XSD schema and subsystem, and added a unit test that verifies the
attributes are read correctly.
MODE-1818 Minor corrections per code review