tor-keymgr: A handful of API fixes

This removes the KeyMgr::*_with_derived() functions.

KeyMgr::generate() now returns the generated key. I'm removing KeyMgr::get_or_generate() (because I'm not sure what its API should look like, and I think it can be added later).

An important point to note is that maybe_generate_hsid no longer stores the KP_hs_id in the key store. The reason we put the KP_hs_id in the keystore in the first place was to support offline HsId mode. However, offline HsId mode was never fully implemented (#1194), and the decision to put the public part of the HsId in the keystore is controversial (#1195 (closed)). We can revisit this decision when we implement #1194, but for now, we don't need a separate KP_hs_id entry in the keystore.

Closes #1074 (closed)