This adds a 'rate' command to modify the up and download
rates on the fly.

Closes GH-10

Closes GH-7

Torrents are now considered stopped and may be restarted even if
the stop event haven't been sent the trackers yet. The same holds
for the del and add commands.

A btpd process in shutdown mode that only have stopped torrents,
but is still sending the stop event to trackers, will release
resources that would block a new btpd to start. It will the
silently exit when it's finished with the trackers.

This also makes the timeout parameter for shutdown unnecessary.

'btcli stop -a'.

make it act properly on iobuf errors.