notmuch.el: Exceute "notmuch search" asynchronously.

Previously, our emacs interface was waiting for the "notmuch search"
to complete before it would display anything. Now, we execute the
process asyncrhonously and filter results as they come in.

This takes advantage of the recent work to make "notmuch search"
results stream out steadily. The result is that some search results
will be available nearly instantly and the user can navigate and view
those while additional results continue loading.