fix #2079: add periodic auth key rotation

fix #2079: add periodic auth key rotation

and modify checks to accept still valid tickets generated using the
previous auth key.

the slightly complicated caching mechanism is needed for reading the key and
its modification timestamp in one go while only reading and parsing it again if
it has changed.

the +- 300 seconds fuzzing is kept for slightly out-of-sync clusters, since the
time encoded in the tickets is the result of time() on whichever node the
ticket API call got forwarded to.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>