wal: fix tx boundaries

In order to preserve transaction boundaries in replication protocol, wal
assigns each tx row a transaction sequence number (tsn). Tsn is equal to
the lsn of the first transaction row.

Starting with commit 7eb4650e, local
space requests are assigned a special replica id, 0, and have their own
lsns. These operations are not replicated.

If a transaction starting with a local space operation ends up in the
WAL, it gets a tsn equal to the lsn of the local space request. Then,
during replication, when such a transaction is replicated, the local
space request is omitted, and replica receives a global part of the
transaction with a seemingly random tsn, yielding an ER_PROTOCOL error:
"Transaction id must be equal to LSN of the first row in the transaction".

Assign tsn as equal to the lsn of the first global row in the
transaction to fix the problem, and assign tsn as before for fully local
transactions.

Follow-up #4114
Part-of #4928

Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>