relay: send rows transactionally

Some time ago we started writing transaction boundaries to WAL and
respecting them in the replication stream: replicas wait for a full
transaction receipt before applying it.

However, during all these changes relay remained transaction-agnostic:
it simply read single rows from WAL and sent them over to the receiver.

This lead to a handful of ugly crutches: for example, tsn is not always
equal to the lsn of the first global row of the transaction: if the
first row is local, tsn is deduced from the first global row of the
transaction.

Also a dummy NOP was appended to the end of a transaction ending by a
local row, so that is_commit flag wasn't lost by the replication.

Let's make relay read a full transaction, filter out all the unnecessary
rows, set the transaction boundaries accordingly and then send the
transaction at once.

Since in relay a single fiber sends data to the remote peer, there is no
chance for a heartbeat to get in between rows of a single transaction:
they're all sent at once. Hence the deletion of a corresponding guard
`relay->is_sending_tx`.

Prerequisite #8958

NO_DOC=internal change
NO_CHANGELOG=internal change
NO_TEST=covered by existing tests

(cherry picked from commit f96782b5)