kvdb+etcd: do not create transaction if the STM write set is empty

This is a small optimization that will skip building of the STM
transaction upon commit when the write set is empty. As we're already
reading a snapshot, transaction formation is useless.
This also prevents retries when doing a kvdb.View and any of the keys
change.

channeldb/kvdb/etcd/stm.go

@@ -711,6 +711,14 @@ func (s *stm) OnCommit(cb func()) {
 // because the keys have changed return a CommitError, otherwise return a
 // DatabaseError.
 func (s *stm) commit() (CommitStats, error) {
+	// Do not create the transaction if the write set is empty.
+	if len(s.wset) == 0 {
+		return CommitStats{
+			Rset: len(s.rset),
+			Wset: 0,
+		}, nil
+	}
+
 	rset := s.rset.cmps(s.lset)
 	wset := s.wset.cmps(s.revision + 1)
 

channeldb/kvdb/etcd/stm_test.go

@@ -334,6 +334,10 @@ func TestManualTxError(t *testing.T) {
 	require.NoError(t, err)
 	require.Equal(t, []byte("xyz"), val)
 
+	// Rewrinting the key/val will make the commit fail due
+	// to the conflicting external put above.
+	stm.Put("123", "abc")
+
 	// Commit will fail with CommitError.
 	err = stm.Commit()
 	var e CommitError