Fixes related to running LDA over a socket in testonly mode, and to Windows usage

Fixed some more issues, this time related to running LDA over a socket in testonly mode, and to using VW on Windows:

- When running as a socket server and using LDA to get predictions, return to the client results from LDA instead of the active learning results.
- Fixed a crash in case the socket connection is dropped (the client hangs up).
- On Windows, sockets are not file descriptors. Therefore, _read, _write, and _close should not be used for sockets. Replaced with recv, send, and closesocket.
- When doing LDA, don't force eta to 10 for "non default update rule". (Otherwise the testonly mode does not work as expected.)

All these were helpful for me, but they might not really comply with the general design of VW, so please feel free to ignore.