salad: add LIGHT(count) method

This commit adds a function that retrieves the number of records stored
in a light hash table and makes light users use it instead of accessing
the light count directly. This gives us more freedom of refactoring the
light internals without modifying the code using it.

Needed for #7192

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring