Eliminate the C wrappers.

BufferedSocket has just an input buffer, and FullyBufferedSocket adds
the output buffer.

Allow creating a closed SocketMonitor instance.

Pipes aren't really pollable on Windows.
GLib emulates polling on pipes using separate reader/writer threads.
This adds significant overhead and keeps a strong dependency on GLib.

socketpair() function is absent on Windows as well.
We implement it here in a loose way.