Add Java HashMap backed Mergeable Long Store

This class is very handy for testing of code that interacts with instances of ```MergeableStore[K, Long]```. For example we use it to test Scalding jobs that use a RedisLongStore in production, as the local java backed version prevents test pollution and makes testing generally easier. Since it has been useful for our internal use case I thought other people might also find it useful. 

The use of ```AtomicLong``` behind the scenes means you still get the associative incremental updates as part of merge. Also since it is just local JVM code you don't have to worry about blocking.