buffer applyCh with up to conf.MaxAppendEntries

This change improves throughput in busy Raft clusters.
By buffering messages, individual RPCs contain more Raft messages.
In my tests, this improves throughput from about 4.5 kqps to about 5.5 kqps.

raft.go

@@ -206,7 +206,7 @@ func NewRaft(conf *Config, fsm FSM, logs LogStore, stable StableStore, snaps Sna
 
 	// Create Raft struct
 	r := &Raft{
-		applyCh:       make(chan *logFuture),
+		applyCh:       make(chan *logFuture, conf.MaxAppendEntries),
 		conf:          conf,
 		fsm:           fsm,
 		fsmCommitCh:   make(chan commitTuple, 128),