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.

Note: this change makes local AAE also use the new BFS exchange. I plan to finish up running a performance test tomorrow and add numbers to this pull request to hopefully show that this new approach has no negative impact on normal AAE.

Sibling pull-requests: basho/riak_core#628, basho/riak_repl#616