FLUID-5542: Queued DataSource

Added a Queued DataSource that can combined with different requestQueues to synchronize, throttle, and debounce requests to a DataSource.

http://issues.fluidproject.org/browse/FLUID-5542

This is an extension of the work started in https://github.com/fluid-project/metadata/pull/43
There's some commentary on this page http://wiki.fluidproject.org/display/fluid/Notes+on+Kettle - at the heading "DATASOURCES AND URLS WITHIN KETTLE"