relay: move gc subscriber creation out of it

GC consumer creation and destroy seemed to only happen in box.cc
with one exception in relay_subscribe(). Lets move it out for
consistency. Now relay can only notify GC consumers, but can't
manage them.

That also makes it harder to misuse the GC by passing some wrong
vclock to it, similar to what was happening in tarantool#10047.

In scope of tarantool#10047

NO_TEST=refactoring
NO_CHANGELOG=refactoring
NO_DOC=refactoring

Author: Gerold103

src/box/box.cc

@@ -4713,6 +4713,26 @@ box_process_subscribe(struct iostream *io, const struct xrow_header *header)
 	}
 	struct vclock start_vclock;
 	box_localize_vclock(&req.vclock, &start_vclock);
+	/*
+	 * Register the replica with the garbage collector.
+	 * In case some of the replica's WAL files were deleted, it might
+	 * subscribe with a smaller vclock than the master remembers, so
+	 * recreate the gc consumer unconditionally to make sure it holds
+	 * the correct vclock.
+	 */
+	if (!replica->anon) {
+		bool had_gc = false;
+		if (replica->gc != NULL) {
+			gc_consumer_unregister(replica->gc);
+			had_gc = true;
+		}
+		replica->gc = gc_consumer_register(&start_vclock, "replica %s",
+						   tt_uuid_str(&replica->uuid));
+		if (replica->gc == NULL)
+			diag_raise();
+		if (!had_gc)
+			gc_delay_unref();
+	}
 	/*
 	 * Send a response to SUBSCRIBE request, tell
 	 * the replica how many rows we have in stock for it,

src/box/relay.cc

@@ -1098,27 +1098,6 @@ relay_subscribe(struct replica *replica, struct iostream *io, uint64_t sync,
 	assert(replica->anon || replica->id != REPLICA_ID_NIL);
 	struct relay *relay = replica->relay;
 	assert(relay->state != RELAY_FOLLOW);
-	/*
-	 * Register the replica with the garbage collector.
-	 * In case some of the replica's WAL files were deleted, it might
-	 * subscribe with a smaller vclock than the master remembers, so
-	 * recreate the gc consumer unconditionally to make sure it holds
-	 * the correct vclock.
-	 */
-	if (!replica->anon) {
-		bool had_gc = false;
-		if (replica->gc != NULL) {
-			gc_consumer_unregister(replica->gc);
-			had_gc = true;
-		}
-		replica->gc = gc_consumer_register(replica_clock, "replica %s",
-						   tt_uuid_str(&replica->uuid));
-		if (replica->gc == NULL)
-			diag_raise();
-		if (!had_gc)
-			gc_delay_unref();
-	}
-
 	if (replica_version_id < version_id(2, 6, 0) || replica->anon)
 		sent_raft_term = UINT64_MAX;
 	relay_start(relay, io, sync, relay_process_row,