It's important that bridgestrap's cache timeout is less than or equal to
rdsys's pluggable transport expiry time.  If bridgestrap expires cache
entries after 24 hours and rdsys wants to re-test bridges after 18
hours, then there's a worst case of a 6 hour window within which
bridgestrap will refuse to re-test rdsys's bridges because they're still
in the cache.  Rdsys will keep bothering bridgestrap because it wants a
more recent test result.

This patch reduces bridgestrap's default cache timeout to 18 hours,
which is the number of hours after which bridges should re-upload their
descriptors to the bridge authority:
https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt?id=c2a584144330239d6aa032b0acfb8b5ba26719fb#n369