REF: significant cleanup of winrm hack.

The main thread loop is much simpler, and we have a WinRMChannel class
that is much closer to ssh channel now.

Still no stdin handling though.
BUG: fix stderr not being flushed when error happens.

We had only one is_done attribute for both streams, and consequently one
of the stream internal buffer was not updated. Each stream has its own
is_done attribute to ensure this does not happen anymore.