box: add pin/unping infrastructure for func cache

There are cases when we need to be sure that a function is not
deleted and/or removed from func cache. For example constraints:
they must hold a pointer to struct func while it's very hard to
determine whether there'a constraint that points to given func.

Implement func pin/unpin for this purpose. You can pin a func to
declare that the func must not be deleted. To have to unpin it
when the func is not needed anymore.

NO_DOC=refactoring
NO_CHANGELOG=refactoring