add munin graphs of server-operation latency

[warner]

We recently added code to collect server-side latency stats (how long it takes from the beginning of remote_get_buckets to just before it returns, etc), and to make these available to the stats-gatherer.

We should also write some munin plugins to graph them over time. This may be useful to see how the server's disk contention is changing over time: with a certain level of traffic, does it take longer to perform a single read operation? Do different local filesystem types (ext3, reiserfs, zfs) run faster than others?