FLOE-227: Queued dataSource requests

Added a queuedDataSource to wrap other dataSources and queue up set requests. This way only one set will attempt to execute at a time, thereby preventing concurrency issues.

http://issues.fluidproject.org/browse/FLOE-227
This method needs to be instrumented too - or at least, there should be a configuration of the wrapper where it is instrumented. It would help @the-t-in-rtf s usecase if the "get" method could also be instrumented to be concurrency-limited.