detect network pushback

[Antoine Martin]

Following #999, the idea here was to use non-blocking sockets and catch EINTR then use this event to detect when the OS is blocking on writes.

The problem is that python3 actually retries syscalls automatically on EINTR so that's a non-starter.

See:

• ​PEP 475 -- Retry system calls failing with EINTR
• ​PEP 475 - EINTR handling

There may be other socket options we can use on Linux: ​socket man page but this won't be portable.

[Antoine Martin]

The only way to do this would be to use native OS calls for the network layer.

[Antoine Martin]

For Linux, TCP_INFO: #2421.

[migration script]

this ticket has been moved to: ​https://github.com/Xpra-org/xpra/issues/2156