Work out how long it takes sbws to measure the network

In the bandwidth file, sbws should report how long it takes to measure (almost all) the entire network.

Here's one way we could work out the network measurement "half-life":

1. Count the number of relays in the network, N
2. Find the last N/2 successful, unique relay measurements
3. Report how much time has elapsed since the earliest measurement in the list (We use the half-life, so that failed measurements don't affect the final time too much.)

We could monitor this figure for each scanner, to make sure they are working correctly.