Since these are arrays instead of maps, the "keys" here are actually
(useless) goto labels. What really matters is that the ordering is
the same between the constant declarations and the array.
Before this commit, compact always compact log at current appliedindex of raft.
This prevents us from doing non-blocking snapshot since we have to make snapshot
and compact atomically. To prepare for non-blocking snapshot, this commit make
compact supports index and nodes parameters. After completing snapshot, the applier
should call compact with the snapshot index and the nodes at snapshot index to do
a compaction at snapsohot index.
send should not attach current term to msgProp. Send should simply do proxy for msgProp without
changing its term. msgProp has a special term 0, which indicates that it is a local message.
The usage of removed:
1. tell removed node about its removal explicitly using msgDenied
2. prevent removed node disrupt cluster progress by launching leader election
It is set when apply node removal, or receive msgDenied.
This prevents the bug like this:
1. a node sends join to a cluster and succeeds
2. it starts with empty peers and waits for sync, but it have not
received anything
3. election timeout passes, and it promotes itself to leader
4. it commits some log entry
5. its log conflicts with the cluster's
Configure is used to propose config change. AddNode and RemoveNode is
used to apply cluster change to raft state machine. They are the
basics for dynamic configuration.
