TODO

List of major "TODO" items left:
 - Transaction manager replay log on restart.  Necessary for txman to come back
   after a crash.
 - Locking in the key-value store.  Necessary to actually uphold serializability.
   This will require implementing SCAN first, otherwise it will need to be
   rewritten.
 - Client-DC affiliation (currently just picks one group at random per
   transaction)
 - Garbage collection of in-memory structures
 - Garbage collection of log
 - SCAN(k, n) operation.  Equivalent to selecting the next n keys >= k.
 - Testing
 - Optimization
    - Durable log throughput/latency
    - Remove early phases of paxos with implicit leader
 - removing paxos group should not crash txmen


- rate limit p1a messages for global/local gpaxos

- retransmit m1b via inner machine
- add assert to ensure learned cstruct never goes lower
- every txman operation should check outcome in dispositions and react
  appropriately
- configuration serial/de-serial
- Don't log commit record repeatedly
- Don't send commit record repeatedly
- change global voter retransmission (take durability into account so a message
  isn't resent before it ever passes the durability barrier).