fix(bigtable): apply one server time to all mutations in a set

This is an important semantic property of BigTable:

  "All mutations in a single write request have the same timestamp
  unless you override them."

  https://cloud.google.com/bigtable/docs/writes#write-types

bigtable/bttest/inmem.go

@@ -1083,6 +1083,9 @@ func (s *server) PingAndWarm(ctx context.Context, req *btpb.PingAndWarmRequest)
 // fam should be a snapshot of the keys of tbl.families.
 // It assumes r.mu is locked.
 func applyMutations(tbl *table, r *row, muts []*btpb.Mutation, fs map[string]*columnFamily) error {
+	// Generate one server timestamp for all mutations in this sequence.
+	serverTime := newTimestamp()
+
 	for _, mut := range muts {
 		switch mut := mut.Mutation.(type) {
 		default:
@@ -1094,7 +1097,7 @@ func applyMutations(tbl *table, r *row, muts []*btpb.Mutation, fs map[string]*co
 			}
 			ts := set.TimestampMicros
 			if ts == -1 { // bigtable.ServerTime
-				ts = newTimestamp()
+				ts = serverTime
 			}
 			if !tbl.validTimestamp(ts) {
 				return fmt.Errorf("invalid timestamp %d", ts)