Outbound limit

Allows to limit the size of buffered data (in EM). It's useful when slow clients can't keep up with the traffic and stack up too much data.

I don't really like that EM keeps it's own separate buffer when the TCP stack already has one but then EM would also be supporting a push-back mechanism when the buffer is full. In any case this is a compromise, disconnect the client when too much data has piled up.