raft: Avoid multiple allocs when merging stable and unstable log

Appending to an empty slice twice could (and often did) result in
multiple allocations. This was wasteful. We can avoid this by performing
a single allocation with the correct size and copying into it.

raft/log.go

@@ -332,8 +332,10 @@ func (l *raftLog) slice(lo, hi, maxSize uint64) ([]pb.Entry, error) {
 	if hi > l.unstable.offset {
 		unstable := l.unstable.slice(max(lo, l.unstable.offset), hi)
 		if len(ents) > 0 {
-			ents = append([]pb.Entry{}, ents...)
-			ents = append(ents, unstable...)
+			combined := make([]pb.Entry, len(ents)+len(unstable))
+			n := copy(combined, ents)
+			copy(combined[n:], unstable)
+			ents = combined
 		} else {
 			ents = unstable
 		}