As 263 paramterizable consistency (ready for review)

So this is an unfortunately large-ish change because I had to inject the default consistency and mapping everywhere, but wanted to leave the defaults in place.

So it's a little ugly.  Appreciate any thoughts as to how to improve.

The intent is to make the default consistency (and any special cases) configurable, so that we can test various combinations more easily when evaluating the performance.  Ideally we would like to change everything to quorum consistency, probably.
+1. Thanks for updating the docstring :)