fixup! Update Proposal #254 with latest circuit padding plans.

the network is being overloaded from padding to such a degree that

padding requests should be ignored:

  * circpad_max_machine_padding_pct

    - The maximum ratio of sent padding traffic to sent non-padding traffic

      (expressed as a percent) to allow on a padding machine before ceasing

      to pad. Ex: 75 means 75 padding packets for every 100

      non-padding+padding packets. This definition is consistent with the

      overhead values in Proposal #265.

  * circpad_machine_allowed_cells

  * circpad_global_max_padding_pct

    - The maximum percent of sent padding traffic out of total traffic

      to allow in a tor process before ceasing to pad. Ex: 75 means

      75 padding packets for every 100 non-padding+padding packets.

      This definition is consistent with the overhead values in Proposal

      #265, though it does not take node position into account.

  * circpad_global_allowed_cells

    - The number of padding cells that must be transmitted before the

      per-machine ratio limit is applied.

  * circpad_max_global_padding_pct

    - The maximum ratio of sent padding traffic to sent non-padding traffic

      (expressed as a percent) to allow globally at a client or relay

      before ceasing to pad. Ex: 75 means 75 padding packets for every 100

      non-padding+padding packets. This definition is consistent with the

      overhead values in Proposal #265.

      global ratio limit is applied.

Additionally, each machine can specify its own per-machine limits for

the allowed cell counters and padding overhead percentages.