Add InternalMerge Command

Add the InternalMerge command, which is replacing the anticipated (but never
implemented) AccumulateTS command.  InternalMerge is similar to a put in that it
writes a single value to a key - however, the supplied value is *merged* into
the existing value instead of replacing it.

Merging is not transactional, and thus it is only exposed internally for usage
in certain operations which require a high write performance (such as the
accumulation of time series data).
RocksDB merges are way more involved than just a read followed by a write, they are really going for high performance here.

A Merge usually will go to memory and not touch the actual key. As more merge requests add up to it, RocksDB will (in-memory) pre-merge those merges (still not touching the actual key), assuming associativity. Only when you actually want to know what the value is (or probably automatically after some time when there's a convenient moment to do so) RocksDB will merge into the stored key and write the update.