Eliminate MaxOnionsPending; replace it with MaxOnionQueueDelay
The right way to set "MaxOnionsPending" was to adjust it until the processing delay was appropriate. So instead, let's measure how long it takes to process onionskins (sampling them once we have a big number), and then limit the queue based on its expected time to finish. This change is extra-necessary for ntor, since there is no longer a reasonable way to set MaxOnionsPending without knowing what mix of onionskins you'll get. This patch also reserves 1/3 of the onionskin spots for ntor handshakes, on the theory that TAP handshakes shouldn't be allowed to starve their speedier cousins. We can change this later if need be. Resolves 7291.