update count API, round 3

[David Bremner]

This obsoletes the remainder of

     id:1419971380-10307-1-git-send-email-david at tethera.net

In the version, I only deprecate the old versions of count_messages
and count threads. This allows breaking the patch up into a series;
hopefully it's easier to review this way.

Patch 3/4 depends on id:1425679073-30439-1-git-send-email-david at tethera.net

I haven't updated the go bindings in this series, because a similar
patch to integrate go into the build/test procecess doesn't exist yet.