Use per-vnode locks to guard concurrent folds on the same partition.
Re-implement handoff to use optional callback in kv_vnode
Use bg-mgr's new configuration switch reader and make VNODE_LOCK macro kv-specific
Address final PR concerns
Merge pull request #769 from basho/cet-bg-mgr-vnode-lock

Integrate Background Manager with Handoff And AAE Tree Rebuilds