Ensure proper rebuilding of hash and menu

Offshoot from #345984: Client-side caching of administration menu:

Ensure the hash properly changes when the menu is rebuilt.

I am absolutely not happy with the current implementation, and after tinkering intensively about it, we should really rebuild whenever hook_menu() is called.

AFAIK, menu_rebuild() is (almost?) always called at the end of system-altering requests, so we basically just need to ensure the _next_ request uses a new hash (i.e. truncate the cache table).