* Separate growth factors for tunnel count and tunnel test time
* Reduce growth factors, so probabalistic throttle would activate
* Square probAccept values to decelerate stronger when far from average
* Create a bandwidth stat with approximately 15-second half life
* Make allowTunnel() check the 1-second bandwidth for overload
before doing allowance calculations using 15-second bandwidth
* Tweak the overload detector in BuildExecutor to be more sensitive
for rising edges, add ability to initiate tunnel drops
* Add a function to seek and drop the highest-rate participating tunnel,
keeping a fixed+random grace period between such drops.
It doesn't seem very effective, so disabled by default
("router.dropTunnelsOnOverload=true" to enable)
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