Windows executors cannot re-register

This issue captures an important (but already resolved) bug due to incorrect inheritance of sockets.

When enabling agent recovery, it was discovered that the executors could not re-register to the new agent. They would send the re-register message, and then fail silently. The agent never received the re-register message.

This turned out to be due to incorrect inheritance semantics of sockets. On POSIX systems, os::cloexec was used to prevent file descriptors (or socket handles, on Windows) from being inherited by child processes. On Windows, we were creating SOCKET handles using the CRT API ::socket, which by default created inheritable socket handles. The subsequent call to os::cloexec to prevent this was a no-op, leaving us leaking socket handles to all child processes, causing the described bug.

The solution was to split net::socket into a POSIX and Windows implementation, where on Windows we use the WinSock 2 API WSASocket, which allows us to create the socket upfront with WSA_FLAG_NO_HANDLE_INHERIT, preventing the leaks. This is somewhat like using O_CLOEXEC on Linux.