relay: move relay_schedule_pending_gc before status update

relay_schedule_pending_gc() is executed after relay status update,
which made perfect sense before we've introduced local spaces rework, making
local space operations use a special instance id: 0.
Relay status update is performed only when the remote instance has
reported a bigger vclock, than its previous one. However, we may have an
entire WAL file filled with local space changes, in which case the
changes won't be transmitted to replica, and it will report the same
vclock as before, postponing the scheduled gc until a non-local row is
created on master.

Fix this by reordering relay_schedule_pending_gc() and relay status
update. In case nothing new is added to pending_gc queue and replica
clock is not updated, relay_schedule_pending_gc() will exit on the first
loop iteration, so it doesn't add an overhead.

Also make relay_schedule_pending_gc() use vclock_compare_ignore0() instead
of plain vclock_compare().

Follow-up #4114