HSFORGET command to clear cached client state for a HS
This is a patch used by the Android app Briar (since October 2014) that we would like to upstream. It adds an
HSFORGET command to the control protocol which clears any cached client state relating to a specified hidden service. This can be used to flush state that's likely to be stale before trying to connect to a hidden service via an unstable network connection (such as a mobile data connection).