+ If a storage times out between the vote and the finish, the ZEO cache
  could get populated with objects that don't make it to the storage
  server.

  A new flag self._midtxn_disconnect gets reset in tpc_begin() and set
  in notifyDisconnected().  If tpc_finish() discovers this flag set, it
  raises a ClientDisconnected error before calling tpc_finish() on the
  server.

+ In tpc_finish() we re-order the calls so that the server's tpc_finish()
  is called (and must succeed) before we update the ZEO client cache.

+ The storage name is now prepended to the sort key, to ensure a
  unique global sort order if storages are named uniquely.

+ Added new tests for the above (checkTimeoutAfterVote,
  checkTimeoutProvokingConflicts, checkSortKey).