parameterize bandwidth cutoffs for Guard and Fast flags
Right now we guarantee the Fast flag to any node with (a hard-coded) 20KB of bandwidth, and guarantee the Guard flag to any node with (also hard-coded) 250KB of bandwidth.
We should turn those into parameters, since the nice people doing Tor network simulations don't want to hack our code between sims when comparing values. We should do it in 0.2.2 so they can do simulations on stable Tor too.
(In a later ticket I'm going to suggest that 20KB has become a dumb value for AuthDirFastGuarantee, but that's a bug, not this feature.)
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