Implement getMultiSlices method for HBase storage backend

Currently `HBaseKeyColumnValueStore` doesn't implement a new multi-slices method `Map> getMultiSlices(MultiKeysQueryGroups 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.).

Start by locating HBaseKeyColumnValueStore and the default getMultiSlices implementation, then trace how MultiKeysQueryGroups and SliceQuery are handled. Decide on an appropriate parallel or grouped execution approach, and consider the multi-slice scenarios described in the issue as the completion criteria.

Written by the indexing model from the issue text.