Added `git annex benchmark` which uses the excellent Criterion to benchmark
parts of git-annex. What I'm interested in benchmarking right now is the
sqlite database that is used to manage v6 unlocked files, but having a
built-in benchmark will probably have other uses later.

The benchmark results were pretty good; queries from the database are
quite fast (60 microseconds warm cache) and scale well as the size increases.
I did find one scalability issue, which was fixed by adding another index
to the database. The kind of schema change that it's easy to make now, but
that would be a painful transition if it had to be done once this was in wide
use.