key manager should re-use tmp keyring

Currently, the key manager creates and destroys a temporary keyring every time an operation is requested using an openpgp key. This might be pretty slow, but maybe it is ok for now.

However, I suspect it will get much slower once we start to encrypt the private key! (related issue #8169)

So, instead of totally destroying the keyring each time, maybe we can keep the temporary keyring around for a longer period of time, perhaps as long as the user is logged in.

(from redmine: created on 2016-06-03, relates #8169)