box: use replicaset.vclock in replica join/subscribe

Again, this is something that was introduced by commit f2bccc18
("Use WAL vclock instead of TX vclock in most places") without any
justification.

TX has its own copy of the current vclock - there's absolutely no need
to inquire it from the WAL thread. Actually, we already use TX local
vclock in box_process_vote(). No reason to treat join/subscribe any
different. Moreover, it's even harmful - there may be a gap at the end
of a WAL file, in which case WAL vclock will be slightly ahead of TX
vclock so that should a replica try to subscribe it would never finish
syncing, see #3830.

Closes #3830