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

[Sebastian Schwarz]

Allow me add to this RFE.

On 2017-02-28, Steven Allen wrote:
> Currently, notmuch fetches keys synchronously.  This can be
> *very* slow (I fetch keys over tor) and locks up emacs for the
> duration.  Therefore, this fetch really should be
> asynchronous.

Even with all keys already present signature verification takes
some time as well.  It would be nice if this was done
asynchronously as well.  This would greatly improve the speed of
opening threads with large numbers of signed messages.