v2 remove notmuch_query_{count,search}_{threads,messages}

[David Bremner]

After some discussion on IRC, here's a second version. This both
removes the old versions, and steals the name.

I've intentionally written notmuch 0.25 in the comments, as I'd like
to merge these changes, or something like them at the beginning of the
0.25 cycle. Origininally I thought that some of the library fixes for
for 0.25 needed API changes, but that doesn't seem to be the case.

d