Fix rebalance in cases where Overflow is invoked

This commit makes the processes redistribute correctly in the case where
there are three nodes, one of which has 2^{M-1} storage processes, and
the large node dies.

Rebalance now proceeds as follows in the case where it cannot take all
processes from the departed node. For Departed node D and predecessor
node P, P.rebalance now:
1. Switches P's processes and backup processes:
   a. Sends a switch  message to each process-backup pair
      (StorateProcessA and Backup{A + 2^{M-1}} for all A between P and D
      modulo 2^{M-1})
   b. Waits to receive confirmations from each that the switching has
      completed
2. Un-registers P's name
3. Re-registers P with D's name
4. Tells its new successor (D's old successor) to re-spawn primary
   copies of all of the storage processes that were converted to backups
   in 1a
5. Waits to receive an acknowledgement from D's old successor
6. Spawns all reaining processes (storage processes and backup
   processes) that were on D, requesting the maps from the appropriate
   nodes
7. Re-enters monitoring.