Add breadth-first AAE exchange

This commit ports the breath-first exchange algorithm from the
synctree code in riak_ensemble to the hashtree code here in riak_core.
This level-by-level exchange is necessary to support streaming and/or
pipelined AAE exchange -- an approach that will soon underpin the new
AAE-based fullsync replication protocol in Riak.

This is the 2.0 rebased version of #628