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.
[minor] What about a timing function that we can use to wrap these other calls?  See http://stackoverflow.com/questions/9160001/how-to-profile-methods-in-scala for a discussion.  It's probably overly complicated when it's only used in two separate routines, but it's fun to think on.