Refactor exit selection process.

The previous commit did not fully fix the issue pointed out in
<https://github.com/NullHypothesis/exitmap/issues/13>.

To fix this, we now only work with cached descriptors except when
we need to look up flags in the network consensus.

Thanks to Arlo for pointing this out.
Drop exit relays which are not in the consensus.

We are only interested in exit relays which made it into the network
consensus.  We ignore all other relays.

Thanks to Arlo for pointing this out.
Merge branch 'issue_13'

Conflicts:
	src/relayselector.py