Address remaining issues in PR
* Remove lock defs file; it was moved to riak_kv_vnode.hrl since all these locks are KV-specific
* The optional call to handoff_started/2 allows vnodes one last chance to abort the handoff process. the function is passed the source vnode's partition number and node name (this node) because the callback does not have access to the full vnode state at this time. In addition the worker pid is passed so the vnode may use that information in its decision to cancel the handoff or not (e.g. get a lock on behalf of the process)
* Move code that calls handooff_started/2 into separate function and put the call inside the try/catch.
Move commonly used configuration switch reader function into core.