Use keycache instead of Signature::Key

* src/mail.cpp (Mail::updateSigstate),
(Mail::getCryptoDetails_o): Use keycache.
* src/parsecontroller.cpp (Parsecontroller::parse): Use
keycache.

This is a huge performance improvement as the old code
was pretty bad. The old code did three keylistings
after a verify to search the key, then check if the
key is secret and then update the key.