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

... and cleaning up on reconfigures
@aranhoide Here is my solution to the draining problem.  I make the verifiedCh big enough to hold all verified masquerades.  That way, workers will never block on writing to it.  That way, I don't have to bother draining it in order to allow the workers to terminate.

Plus, this keeps the workers from potentially blocking if nobody is using the verified masquerades.