Implement getMultiSlices method for HBase storage backend

[porunov]

Currently HBaseKeyColumnValueStore doesn't implement a new multi-slices method Map<SliceQuery, Map<StaticBuffer, EntryList>> getMultiSlices(MultiKeysQueryGroups<StaticBuffer, SliceQuery> multiKeysQueryGroups, StoreTransaction txh).
It means that HBase storage backend executes each Slice query sequentially instead of doing so in parallel.
Thus, the performance of using HBase storage backend for multi-slice queries might be affected.

Ideally HBaseKeyColumnValueStore would need to overwrite a default getMultiSlices implementation with the one which queries each Slice query in parallel using either a thread pool, asynchronous execution, or grouping of multiple slices together.
If so, HBase performance should quite improve for multi-slice scenarios (i.e. for queries like .valueMap("foo", "bar"), .has("foo", "bar").has("bar", "foo"), out("foo", "bar"), etc.).