Move unix_time(), id_cpy()/id_eq(), is_timeout() to util.*

unix_time():
- returns local value for current epoch
- value is updated explicitly with unix_time_update() called at new_DHT()/doMessenger()/do_DHT()

is_timeout():
- uses the local value for current epoch

id_cpy()/id_eq() => id_copy()/id_equal():
- centralized duplicate definitions
- replaced (most) memcpy()/memcmp() of (*, *, CLIENT_ID_SIZE) with id_copy()/id_equal()