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.

src/util/time.cpp

@@ -123,3 +123,8 @@ struct timeval MillisToTimeval(int64_t nTimeout)
     timeout.tv_usec = (nTimeout % 1000) * 1000;
     return timeout;
 }
+
+struct timeval MillisToTimeval(std::chrono::milliseconds ms)
+{
+    return MillisToTimeval(count_milliseconds(ms));
+}