Backport 2.0.1 AAE improvements

This is a prerequisite for the backporting of a number of MDC
improvements done in the 2.0.X series to prepare a 1.4.11 release.
This version passes the AAE riak_test.

The original changes come from PRs against both riak_core and riak_kv.
But on the 1.4 branch, the hashtree module is still in riak_kv, so they are
both included here.

basho/riak_core#629
basho/riak_kv#1026

/cc @jonmeredith 
it takes it from kv and default to an entry in core actually.  With the movement of hashtree between repos that should allow either.