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.