Measure how often tor clients fetch the consensus

We have an idea of how often this should be happening but we don't have (as far as I can see) any data on how often this actually is happening.

This is important because the network team may use the data to identify bugs and reduce load, and because our user count data is based on directory fetches and if we have the wrong number here, then our user count is wrong.

This could be achieved by logging fetches from onionperf, where we already have a long running tor daemon, or we could run daemons independently. Running them independently would give us the ability to track how often fetches occur for different versions of tor.