replica: advance gc state only when xlog is closed

Advancing replica->gc on every status update is inefficient as gc can
only be invoked when we move to the next xlog file. Currently, it's
acceptable, because status is only updated once per second, but there's
no guarantee that it won't be updated say every millisecond in future,
in which case advancing replica->gc on every status update may become
too costly.

So introduce a trigger invoked every time an xlog is closed by
recover_remaining_wals() and use it in relay to send a special gc
message.