pgp: GpgmeKeyring: don't load private and public keys simultaneously
When trust-model tofu or tofu-pgp is used, loading both at the same time hits a deadlock in GnuPG: https://dev.gnupg.org/T3748 (discovered in context of GPA).
Upstream never fixed it properly, instead opting for the workaround in form of loading private and public keys sequentially. Let's do the same.
Fixes #2 (closed).
Edited by Ivan Shapovalov