Clean-up: hash table utils are unnecessarily non-const

Clean-up: hash table utils are unnecessarily non-const

Non-const internal utilities make it hard/impossible to write
const-correct code in the rest of the project.

This clean-up allows the notification subsystem to improve on
this code-quality front (see follow-up patches).

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>