Query stats - comments please

Adding an option to the data store config (default true) to collect query stats. Added instrumentation into the AccumuloFeatureReader.
The persisting of stats is done in an asynchronous background thread - not all stats are guaranteed to be written, but in exchange it consumes less resources.
The initial instrumentation is fairly light, as I didn't want to pollute the code too much. Considered using aspects, but it seemed like overkill at the moment - plus a lot of the underlying methods we'd be intercepting are undergoing change.
Yeah, the idea is that in the AccumuloDataStoreFactory we can do:
new AccumuloDataStore with StatWriter
if stats are enabled and then we don't need to change anything in AccumuloDataStoreFactory... does that make sense?