Like `check_own_socket_thread`, we can move the functionality of monitoring parent and homedir into a thread (from `handle_tick`).
By doing so, it will be:
- possible: change the interval value at compile time independent to `TIMERTICK_INTERVAL`
- improvement: the need for check (thus, the need to spawn the thread) can be determined once, and no check each `handle_tick`
- possible: `handle_tick` can be removed and `agent_cache_housekeeping` can be improved to support exact timeout