sbws bandwidth scans should require a minimum exit bandwidth

When sbws is constructing a two-hop measurement circuit to run a test, it tries to pick an exit that has at least twice the consensus weight of the current relay-under-test: https://github.com/torproject/sbws/blob/master/sbws/core/scanner.py#L216

So this means that in this case, sbws would have picked any exit that was not a BadExit, has an acceptable ExitPolicy, and has a consensus weight of at least, well, 2. That's not a lot.

As it turns out, something like 10% of exits have under a 600Kbyte/sec advertised bandwidth. So it seems pretty easy from this weight=1 bootstrap scenario to get paired with an exit that will give poor test results.

Perhaps bwauth path selection should also choose a testing pair from exits/relays with a certain absolute minimum of weight or advertised bandwidth?

Reported by Jimmy on tor-relays: https://lists.torproject.org/pipermail/tor-relays/2020-January/018027.html