The behavior accelarates the happen of the first-time leader election,
so the cluster could elect its leader fast. Technically, it could
help to reduce `electionMs - heartbeatMs` wait time for the first leader election.
Main usage:
1. Quick start for the local cluster when setting a little longer
election timeout
2. Quick start for the global cluster, which sets election timeout to
its maximum 50s.
When it waits for apply to be done, it should stop the loop if it
receives stop signal.
This helps to print out panic information. Before this PR, if the panic
happens when server loop is applying entries, server loop will wait for
raft loop to stop forever.
After this PR, only cluster's interface Cluster is exposed, which makes
code much cleaner. And it avoids external packages to rely on cluster
struct in the future.
The PR extracts types.Cluster from etcdserver.Cluster. types.Cluster
is used for flag parsing and etcdserver config.
There is no need to expose etcdserver.Cluster public, which contains
lots of etcdserver internal details and methods. This is the first step
for it.
Stop raftNode goroutine before stopping server goroutine, so
server.Stop does stop all underlying stuffs elegantly now. This fixes
the problem that previous-round lock on WAL may not be released when
etcd is restarted.
Do not aggressively compact raft log entries. After a snapshot,
etcd server can compact the raft log upto snapshot index. etcd server
compacts to an index smaller than snapshot to keep some entries in memory.
The leader can still read out the in memory entries to send to a slightly
slow follower. If all the entries are compacted, the leader will send the
whole snapshot or read entries from disk if possible.
