Acceptor state garbage collection

[nruth]

Acceptor state grows linearly and results in measured throughput drop and latency increase.

Implement simple garbage collection as in Renesse's "Paxos Made Moderately Complex", making use of having 2f+1 replicas.

So far partially complete:
Acceptors can handle a gc request, but someone else (proposer / commander) needs to track replica state and send the gc request appropriately.