salad: add LIGHT(random) method

This commit moves the code that gets the index of a random light
record from the memtx hash index implementation to a new light method.
This gives us more freedom of refactoring the light internals without
modifying the code using it.

After this change, LIGHT(pos_valid) isn't needed anymore so it's
inlined in LIGHT(random).

Needed for #7192

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit 76add786)