vinyl: allow to commit statements to mem in arbitrary order

vy_mem_commit_stmt() expects statements to be committed in the order
of increasing LSN. Although this condition holds now, it won't once
we start using this function for building indexes. So let's remove
this limitation.

Needed for #1653

src/box/vy_mem.c

@@ -245,11 +245,14 @@ vy_mem_commit_stmt(struct vy_mem *mem, const struct tuple *stmt)
 	/* The statement must be from a lsregion. */
 	assert(!vy_stmt_is_refable(stmt));
 	int64_t lsn = vy_stmt_lsn(stmt);
-	if (mem->min_lsn == INT64_MAX)
-		mem->min_lsn = lsn;
-	assert(mem->min_lsn <= lsn);
-	if (mem->max_lsn < lsn)
-		mem->max_lsn = lsn;
+	/*
+	 * Normally statement LSN grows monotonically,
+	 * but not in case of building an index on an
+	 * existing non-empty space. Hence use of MIN/MAX
+	 * here.
+         */
+	mem->min_lsn = MIN(mem->min_lsn, lsn);
+	mem->max_lsn = MAX(mem->max_lsn, lsn);
 	/*
 	 * If we don't bump mem version after assigning LSN to
 	 * a mem statement, a read iterator which uses