Tweak --drop-guards switch.
This commit tweaks the recently added --drop-guards switch as follows: - Guards are dropped right at startup and then every N hours. Otherwise we might not receive the first round of GUARD NEW/UP events. It's unclear why we don't receive those events, but finding out might be time-consuming whereas dropping guards at startup is basically free. - Right after guards are dropped, circuit build timeouts are dropped, too, if supported by the Tor version. If the Tor version does not support this, there's going to be a warning, and the control log will simply not contain BUILDTIMEOUT_SET events. Still part of the reopened tpo/metrics/onionperf#33399.