Update to use new breadth-first AAE exchange

This commit updates riak_kv to work with the new breath-first AAE
exchange added to riak_core. This new level-by-level exchange is
necessary in order to support the new pipelined AAE-based fullsync
replication protocol in Riak Enterprise.
Merge pull request #1026 from basho/feature/aae-exchange-bfs-2.0

Update to use new breadth-first AAE exchange

Reviewed-by: jonmeredith