revision tracking patches, round 1

[David Bremner]

Since the previous WIP posting 

     id:1428274754-1698-10-git-send-email-david at tethera.net

I've reorganized a bit and added some new tests, and an option to
notmuch-count to output the global database revision.  I think the
first 6 patches are probably ready for serious review at this point.

The remaining patches in the series make the modification count more
useful (mainly by including it in structured output formats), but
these are already enough for library users (such as Gaute, who has
reported success running them).