If any client currently displaying a window pane has more than 1 KB of output
buffered, don't accept any further data from the process running in the pane.
This makes tmux much more responsive when flooded with output, although other
buffers can still have an impact when running remotely.
Prompted by a query from Ranganathan Sankaralingam.
Split the server code handling clients, jobs and windows off into separate
files from server.c (merging server-msg.c into the client file) and rather than
iterating over each set after poll(), allow a callback to be specified when the
fd is added and just walk once over the returned pollfds calling each callback
where needed.
More to come, getting this in so it is tested.
