Load a counter without necessarily reading it.

In the case of counters, it's useful to not always have to read in a value prior to writing.  This can reduce volume on clusters & improve throughput.  I've verified that this is not strictly required as per the responses on the riak mailing list.

I'm not married to structuring the code this way, but I didn't want to start doing any refactoring without prior approval.  I figured I'd point out the change and rejigger it however you'd prefer.