etcdserver: increase maxGapBetweenApplyAndCommitIndex

This exists to prevent sending too many requests that
would lead into applier falling behind Raft accepting-proposal.

Based on recent benchmarks, etcd was able to process high workloads
(2 million writes with 1K concurrent clients).

The limit 1000 is too conservative to test those high workloads.

etcdserver/v3_server.go

@@ -47,7 +47,7 @@ const (
 	// the applied index and committed index.
 	// However, if the committed entries are very heavy to apply, the gap might grow.
 	// We should stop accepting new proposals if the gap growing to a certain point.
-	maxGapBetweenApplyAndCommitIndex = 1000
+	maxGapBetweenApplyAndCommitIndex = 5000
 )
 
 var (