Use LOCKMODE in parse_relation.c/.h

There are a couple of comments in parse_relation.c

> Note: properly, lockmode should be declared LOCKMODE not int, but that
> would require importing storage/lock.h into parse_relation.h. Since
> LOCKMODE is typedef'd as int anyway, that seems like overkill.

but actually LOCKMODE has been in storage/lockdefs.h for a while,
which is intentionally a more narrow header. So we can include that
one in parse_relation.h and just use LOCKMODE normally.

An alternative would be to add a duplicate typedef into
parse_relation.h, but that doesn't seem necessary here.