net: add RAII socket and use it instead of bare SOCKET
Introduce a class to manage the lifetime of a socket - when the object
that contains the socket goes out of scope, the underlying socket will
be closed.
In addition, the new `Sock` class has a `Send()`, `Recv()` and `Wait()`
methods that can be overridden by unit tests to mock the socket
operations.
The `Wait()` method also hides the
`#ifdef USE_POLL poll() #else select() #endif` technique from higher
level code.
