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).
I've augmented the comment on the "Engine.Merge()" method, which seemed like the most central place to put this.  I've added a note here in internal.go pointing to that case.