Make it possible to forget bridge descriptors before SETCONF

The anti-censorship team maintains bridgestrap which does bridge testing for us. Bridgestrap tests bridges by SETCONFing them using tor's control port. Tor's caching gets in the way here, so to repeatedly test bridges, it's helpful for tor to be able to forget bridge descriptors. @arma wrote a patch for that in this comment. It would be convenient for us to have that functionality directly in tor, so I wonder if it would be sensible to merge Roger's patch and expose it via a config option?