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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 #33399.

And clarify that we're leaving statistics unchanged as part of the
filtering.

 - Add a change log entry.
 - Pick are more sensible default for `drop_guards_interval_hours`,
   also to fix unit tests.

Implements #33260.

 - Change timeout back to 270 seconds and stallout back to 0 seconds.
 - Change initial pause to 300 seconds to keep default behavior
   unchanged.
 - Change model, so that pause_between starts in parallel to a stream,
   not when the stream is completed. This is the same behavior as
   before.

Also add a change log entry for all changes.