Add MergeableJMapLongStore and accompanying tests

This class is very handy for testing code that interacts with a
MergeableStore[K, Long] instance.
Refactor for more idomatic Scala
Remove use of AtomicLong and make thread-safe

Place a lock around the merge operation to ensure the merge
operation is thread-safe. Use a java.util.concurrent.ConcurrentHashMap
as the backing store to make get/put threadsafe
Merge pull request #146 from sharethrough/add_jmap_backed_mergeable_long_store

Add Java HashMap backed Mergeable Long Store