replication: promote tx vclock only after successful wal write

Applier used to promote vclock prior to applying the row. This lead to
a situation when master's row would be skipped forever in case there is
an error trying to apply it. However, some errors are transient, and we
might be able to successfully apply the same row later.

While we're at it, make wal writer the only one responsible for
advancing replicaset vclock. It was already doing it for rows coming
from the local instance, besides, it makes the code cleaner since now we
want to advance vclock direct from wal batch reply and lets us get rid of
unnecessary checks whether applier or wal has already advanced the
vclock.

Closes #2283
Prerequisite #980