Implemented congestion control

Implemented congestion control

Servers are allowed to send 100 cells initially, and can't send more until
they receive a 'sendme' cell from that direction, indicating that they
can send 10 more cells. As it currently stands, the exit node quickly
runs out of window, and sends bursts of 10 whenever a sendme cell gets
to him. This is much much much faster (and more flexible) than the old
"give each circuit 1 kB/s and hope nothing overflows" approach.

Also divided out the connection_watch_events into stop_reading,
start_writing, etc. That way we can control them separately.

svn:r54