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

... and cleaning up on reconfigures
Also, how often do we typically get a call to Configure?  Are all verifiedMasqueradeSets invalidated and reverified every time this happens?  That would feel a bit wasteful , esp. in combination with connection pooling, which may make us eagerly pre-verify more than we actually need.  OTOH it's perhaps cleanest too.

Oh well, let's see if we observe a performance issue in testing.