get back on upstream yubihsm crate

[flihp]

In the run up to the last ceremony we ended up in a situation where the YubiHSM session space was getting exhausted. There had been some back & forth upstream and I can't remember why but the code to cleanup sessions in the YubiHSM connection object on drop got backed up for whatever reason. So we were creating and destroying connection objects at a rate sufficient to exhaust the available sessions. This left us to clean up these sessions ourselves, but we didn't get enough access to the inner workings of the connection to close these sessions manually. So we hacked up something ... I can't remember if this went upstream or not. This is another task that falls under #222