box: do not rotate WAL when replica subscribes

Because this is pointless and confusing. This "feature" was silently
introduced by commit f2bccc18 ("Use WAL vclock instead of TX vclock
in most places"). Let's revert this change. This will allow us to
clearly separate WAL checkpointing from WAL flushing, which will in turn
facilitate implementation of the checkpoint-on-WAL-threshold feature.

There are two problems here, however. First, not rotating the log breaks
expectations of replication/gc test: an xlog file doesn't get deleted in
time as a consequence. This happens, because we don't delete xlogs
relayed to a replica after join stage is complete - we only do it during
subscribe stage - and if we don't rotate WAL on subscribe the garbage
collector won't be invoked. This is actually a bug - we should advance
the WAL consumer associated with a replica once join stage is complete.
This patch fixes it, but it unveils another problem - this time in the
WAL garbage collection procedure.

Turns out, when passed a vclock, the WAL garbage collection procedure
removes all WAL files that were created before the vclock. Apparently,
this isn't quite correct - if a consumer is in the middle of a WAL file,
we must not delete the WAL file, but we do. This works as long as
consumers never track vlcocks inside WAL files - currently they are
advanced only when a WAL file is closed and naturally they are advanced
to the beginning of the next WAL file. However, if we want to advance
the consumer associated with a replica when join stage ends (this is
what the previous paragraph is about), it might occur that we will
advance it to the middle of a WAL file. If that happens the WAL garbage
collector might remove a file which is actually in use by a replica.
Fix this as well.