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
Now we have this compact expression, it's worth filling it out with a few more suggestive examples - I would think that sss, lss, ssl, lsss, ssss and sssl would be worth trying.