Windows: wait_{read,writ}able limited to 512 sockets

[njsmith]

This is because we currently use select.select. The limit is entirely artificial -- the underlying fd_set object is just a (int length, array of SOCKET integers) structure which we could allocate at whatever size we wanted, if we wanted. Or we could use WSAPoll.

[njsmith]

Though note that WSAPoll is quirky:

• https://social.msdn.microsoft.com/Forums/windowsdesktop/en-US/18769abd-fca0-4d3c-9884-1a38ce27ae90/wsapoll-and-nonblocking-connects-to-nonexistent-ports?forum=wsk
• https://daniel.haxx.se/blog/2012/10/10/wsapoll-is-broken/

[njsmith]

This should be considered in combination with #52.

[oremanj]

I think this is superseded by the plan in #52 to stop using select() entirely.