Rename txn->{n_local_rows,n_remote_rows}

We need to keep count of different kind of rows in a transaction:
- rows which already exist in some WAL and are replayed locally.
  These used to be called n_remote_rows, and renamed to n_applier_rows
- rows which were created locally, on this server (previously called
  n_local_rows, renamed to n_new_rows).

Of the latter, we need to distinguish between GROUP_ID=LOCAL rows,
i.e. rows which need not be replicated, and GROUP_ID=REPLICA rows,
which need to be replicated. For example, a remote transaction can
fire local triggers which generate local rows. If these triggers
generate rows which need to be replicated, the transaction has to be
aborted.

In a subsequent patch I plan to add n_local_rows, which tracks the
number of new rows with GROUP_ID=local and use it in
txn_is_distributed() check.