replication: fix downstream lag growing when there's no new transactions

downstream lag is the difference in time between the moment a
transaction was written to master's WAL and the moment an ack for it
arrived.

Its calculation is supported by replicas sending the last applied row
timestamp. When there is no replication, the last applied row timestamp
stays the same, so in this case downstream lag grows as time passes.

Once an old master is replaced by a new one, it notices changes in peer
vclocks and tries to update downstream lag unconditionally. This makes
the lag appear to be growing indefinitely, showing the time since the
last transaction on the old master:

```
 downstream:
   status: follow
   idle: 0.018218606001028
   vclock: {1: 3, 2: 2}
   lag: 34.623061401367
```

The commit 56571d83 ("raft: make followers notice leader hang")
made relay exchange information with tx even when there are no new
transactions, so the issue became even easier to reproduce.

The issue itself was present since downstream lag introduction in commit
29025bce ("relay: provide information about downstream lag").

Closes #7581

NO_DOC=bugfix