Remove hard-coded thread pool size for Async tasks.

Pull Request: https://github.com/zanata/zanata-server/pull/643
Squashed commit of the following:

commit a772b78b841a7ec374d4ee9674510b3d73e6e3db
Author: Carlos A. Munoz <camunoz@redhat.com>
Date:   Mon Dec 8 16:35:45 2014 +1000

    Code review changes.

commit d43bb366ed753b960374b3bbd565638e10a34aed
Author: Carlos A. Munoz <camunoz@redhat.com>
Date:   Wed Dec 3 17:09:39 2014 +1000

    Remove hard-coded thread pool size for Async tasks.

    Made it configurable via system properties, and added some infrastructure classes to implement different types of configuration stores. All configuration stores should migrate to this design in the future.