Refactored handoff sender max_concurrency to reduce overloaded meanings.

Addresses #564 [JIRA](https://bashoeng.atlassian.net/browse/RIAK-1189) — still needs tests, just opening this to get review started.

* Separated handoffs terminated for concurrency limit violations from those
   terminated for real errors -- concurrency violations no longer cause
   abnormal process exits.

 * Split up the three sources of max_concurrency exits in handoff_sender:
     1. vnode lock max_concurrency
     2. remote node hitting its concurrency limit
     3. concurrency limit lowered after handoffs already started

 * Added a mechanism for marking a transfer as "rescheduled" -- considered
   failed on completion.

   This is used to notify the handoff manager that a given handoff, while
   exiting normal, should not signal xfer_complete to the corresponding
   vnode. NB this logic is basically working around the relatively tight
   coupling between handoff_manager and vnode_manager; while there's some
   groundwork laid for decoupling them in the form of the optional
   `handoff_started/2', the handoff manager still special-cases a bit
   just for vnode_manager.
After talking about this on the call I'm inclined to just ignore this case and not catch it — if/when we get back to background_mgr we can clean it up, since this is sort of out of scope.