vinyl: rotate mem during index build on demand

Meanwhile in 17f6af7 the similar problem has been fixed, still it may
appear that in-memory level of secondary index being constructed has
lagging memory generation (in other words, it's values is less than
the value of current global generation). Such situation can be achieved
if yield which is required to check uniqueness of value being inserted
is too long. In this time gap other space may trigger dump process
bumping global memory generation counter, but dump itself is still not
yet scheduled.

So to get rid of generations mismatch, let's rotate in-memory level
after yield on demand.

Note that test is not included into the patch due to its complexity
and will be added as a follow-up.

Closes #5042

(cherry picked from commit c3561b1)

src/box/vinyl.c

@@ -4097,6 +4097,12 @@ vy_build_insert_tuple(struct vy_env *env, struct vy_lsm *lsm,
 	 * Hence, to get right mem (during mem rotation it becomes
 	 * sealed i.e. read-only) we should fetch it from lsm again.
 	 */
+	if (lsm->mem->generation != *lsm->env->p_generation) {
+		if (vy_lsm_rotate_mem(lsm) != 0) {
+			tuple_unref(stmt);
+			return -1;
+		}
+	}
 	mem = lsm->mem;
 
 	/* Insert the new tuple into the in-memory index. */