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).
Every command is replicated, including non-transactional changes.  

Time series data is always ordered because every piece of data is paired with a timestamp.  The individual samples can be written out-of-order, and that ordering can be easily corrected by the client when the data is read.  There are concerns regarding dropped writes, but monitoring systems are designed to account for this possibility.