applier: fix upstream.lag calculations

upstream.lag is the delta between the moment when a row was written to
master's journal and the moment when it was received by the replica.
It's an important metric to check whether the replica has fallen too far
behind master.

Not all the rows coming from master have a valid time of creation. For
example, RAFT system messages don't have one, and we can't assign
correct time to them: these messages do not originate from the journal,
and assigning current time to them would lead to jumps in upstream.lag
results.

Stop updating upstream.lag for rows which don't have creation time
assigned.

The upstream.lag calculation changes were meant to fix the flaky
replication/errinj.test:

 Test failed! Result content mismatch:
 --- replication/errinj.result	Fri Aug 13 15:15:35 2021
 +++ /tmp/tnt/rejects/replication/errinj.reject	Fri Aug 13 15:40:39 2021
 @@ -310,7 +310,7 @@
  ...
  box.info.replication[1].upstream.lag < 1
  ---
 -- true
 +- false
  ...

But the changes were not enough, because now the test
may see the initial lag value (TIMEOUT_INFINITY).
So fix the test as well by waiting until upstream.lag becomes < 1.