[RFE] Call `gpg --recv-keys` asynchronously

[Sebastian Schwarz]

On 2017-02-28, David Bremner wrote:
> FWIW, these changes are pretty unrelated at an implimentation
> level.  The key fetching is done from emacs, while the
> verification is done in the notmuch binary.

I presumed the verification was done in Emacs as well.  But I
can see how asynchronous signature verification would basically
impossible without large scale refactoring, when it's done by
notmuch itself.