raft: avoid allocation of Raft entry due to logging

`raftpb.Entry.String` takes a pointer receiver, so calling it
on a loop variable was causing the variable to escape. Removing
the `.String()` call was enough to avoid the allocation, but
this also avoids a memory copy and prevents similar bugs.

This was responsible for 11.63% of total allocations in an
experiment with https://github.com/nvanbenschoten/raft-toy.

raft/raft.go

@@ -1005,7 +1005,8 @@ func stepLeader(r *raft, m pb.Message) error {
 			return ErrProposalDropped
 		}
 
-		for i, e := range m.Entries {
+		for i := range m.Entries {
+			e := &m.Entries[i]
 			if e.Type == pb.EntryConfChange {
 				if r.pendingConfIndex > r.raftLog.applied {
 					r.logger.Infof("propose conf %s ignored since pending unapplied configuration [index %d, applied %d]",