Bandwidth capping

[Roger Dingledine]

Currently the tor process (when acting as a router) caps incoming
bandwidth on each connection, based on the 'bandwidth' item in the router
entry. So a tor process with 5 connections, if they each negotiated
to 100KB/s, would try to use as much as 500KB/s if each connection is
saturated. Each new connection means a potential increase in overall
bandwidth.

It seems clear we want an 'overall incoming bandwidth' value. The code
is already set up to round-robin among the connections (it reads at most
10 cells from a connection before going to the next one), so it would
be an easy addition. My current plan is to use the same 'bandwidth'
variable as an overall cap as well: so we leave the code in for the
per-connection bandwidth throttling if we ever find a need for it (and
to handle connections which want a bandwidth less than we want), but we
also do an overall throttling of that same amount.

--Roger