replication: correctly check for rows to skip in applier

Fix replicaset.applier.vclock initialization issues: it wasn't
initialized at all previously. Moreover, there is no valid point in code
to initialize it, since it may get stale right away if new entries are
written to WAL. So, check for both applier and replicaset vclocks.
The greater one protects the instance from applying the rows it has
already applied or has already scheduled to write.
Also remove an unnecessary aplier vclock initialization from
replication_init().

Closes #4739

src/box/applier.cc

@@ -731,8 +731,18 @@ applier_apply_tx(struct stailq *rows)
 	struct latch *latch = (replica ? &replica->order_latch :
 			       &replicaset.applier.order_latch);
 	latch_lock(latch);
-	if (vclock_get(&replicaset.applier.vclock,
-		       first_row->replica_id) >= first_row->lsn) {
+	/*
+	 * We cannot tell which vclock is greater. There is no
+	 * proper place to initialize applier vclock, since it
+	 * may get stale right away if we write something to WAL
+	 * and it gets replicated and then arrives back from the
+	 * replica. So check against both vclocks. Replicaset
+	 * vclock will guard us from corner cases like the one
+	 * above.
+	 */
+	if (MAX(vclock_get(&replicaset.applier.vclock, first_row->replica_id),
+		vclock_get(&replicaset.vclock, first_row->replica_id)) >=
+	    first_row->lsn) {
 		latch_unlock(latch);
 		return 0;
 	}

src/box/replication.cc

@@ -93,7 +93,6 @@ replication_init(void)
 	latch_create(&replicaset.applier.order_latch);
 
 	vclock_create(&replicaset.applier.vclock);
-	vclock_copy(&replicaset.applier.vclock, &replicaset.vclock);
 	rlist_create(&replicaset.applier.on_rollback);
 	rlist_create(&replicaset.applier.on_commit);