add modify-mutable-slot performance test

[warner]

we have an automated test (run by the buildbot, graphed by munin) to tell us how long it takes to create mutable slots (currently about 1.5s each). But we don't have a test to tell us how long it takes us to *modify* these slots. The creation is slowed down by the need to create an RSA keypair for each one. The modification is probably limited by the time to perform an RSA signature operation, or maybe by the roundtrips involved in updating all the slots.. who knows. But in real-world usage, the rate of directory modification is certainly a lot higher than the rate of directory creation, so we should have a test to tell us how long it will take.

[warner]

this is now in place. We track and graph slot creation separately from slot read and slot write. All three have per-operation latencies tracked, and read+write also have per-byte rates.