raft: Set BatchApplyCh for more consistent batch sizes

changelog/11907.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+raft: Improve raft batch size selection
+```

physical/raft/raft.go

@@ -663,6 +663,11 @@ func (b *RaftBackend) applyConfigSettings(config *raft.Config) error {
 	config.NoSnapshotRestoreOnStart = true
 	config.MaxAppendEntries = 64
 
+	// Setting BatchApplyCh allows the raft library to enqueue up to
+	// MaxAppendEntries into each raft apply rather than relying on the
+	// scheduler.
+	config.BatchApplyCh = true
+
 	return nil
 }