Switch to .app.src, add xref options to rebar.config
Integrate riak_core with riak_ensemble
Fix/extend vnode handoff/forwarding behavior

Fix issue where handoff_cancelled was not called on vnode modules
if handoff failed to start due to concurrency limit.

Add new optional set_vnode_forwarding callback that vnode modules
can implement to track riak_core's implicit forwarding behavior
that occurs between a primary handoff finishing and actual ownership
being changed.

All of the above changes are necessary for riak_kv_vnode to properly
support strongly consistent operations during handoff and ownership
transitions.
Merge branch 'jdb-strong-1.4.2' into jdb-strong-2.0-wip

Conflicts:
	rebar.config
	src/riak_core.app.src
	src/riak_core_bucket.erl
	src/riak_core_claimant.erl
	src/riak_core_sup.erl
	src/riak_core_vnode.erl
Disable ensembles subsystem by default
Switch riak_ensemble to jdb-wip-2.0 for tech preview
Merge pull request #441 from basho/jdb-strong-2.0