vinyl: handle multi-statement recovery txns

During recovery and xlog replay vinyl skips the statements already
stored in runs. Indeed, their re-insertion into the mems would
lead to their second dump otherwise.

But that results into an issue that the recovery transactions in
vinyl don't have a write set - their tx->log is empty. On the
other hand they still are added to the write set (xm->writers).
Probably so as not to have too many checks "skip if in recovery"
all over the code.

It works fine with single-statement transactions, but would break
on multi-statement transactions. Because the decision whether
need to add to the write set was done based on the tx's log
emptiness. It is always empty, and so the transaction could be
added to the write set twice and corrupt its list-link member.

The patch makes the decision about being added to the write set
based on emptiness of the list-link member instead of the log so
it works fine both during recovery and normal operation.

Needed for #5874

src/box/vy_tx.c

@@ -899,8 +899,15 @@ vy_tx_begin_statement(struct vy_tx *tx, struct space *space, void **savepoint)
 	}
 	assert(tx->state == VINYL_TX_READY);
 	tx->last_stmt_space = space;
-	if (stailq_empty(&tx->log))
+	/*
+	 * When want to add to the writer list, can't rely on the log emptiness.
+	 * During recovery it is empty always for the data stored both in runs
+	 * and xlogs. Must check the list member explicitly.
+	 */
+	if (rlist_empty(&tx->in_writers)) {
+		assert(stailq_empty(&tx->log));
 		rlist_add_entry(&tx->xm->writers, tx, in_writers);
+	}
 	*savepoint = stailq_last(&tx->log);
 	return 0;
 }