getlantern/lantern#1930 Limiting parallelism of masquerade verification,...

... and cleaning up on reconfigures
1. I need to think about the issue with users of the old verifiedSet.  Will let you know.

2. I  thought about the cost of reverifying too.  The main reason that I do it is that the results depend not just on the masquerade set but also the top QOS server that's used to verify them.  That potentially changes when we reconfigure the client, so rather than trying to get overly sophisticated I decided to just reverify unless it becomes an issue.