gc: do not use WAL watcher API for deactivating stale consumers

The WAL thread may delete old WAL files if it gets ENOSPC error.
Currently, we use WAL watcher API to notify the TX thread about it so
that it can shoot off stale replicas. This looks ugly, because WAL
watcher API was initially designed to propagate WAL changes to relay
threads and the new event WAL_EVENT_GC, which was introduced for
notifying about ENOSPC-driven garbage collection, isn't used anywhere
else. Besides, there's already a pipe from WAL to TX - we could reuse it
instead of opening another one.

If we followed down that path, then in order to trigger a checkpoint
from the WAL thread (see #1082), we would have to introduce yet another
esoteric WAL watcher event, making the whole design look even uglier.
That said, let's rewrite the garbage collection notification procedure
using a plane callback instead of abusing WAL watcher API.