Plumb the SMB2 front end into the blocking lock backend.

Plumb the SMB2 front end into the blocking lock backend.

Metze, you'll probably be happier with this work as it
doesn't abuse tevent in the way you dislike. This is a
first cut at the code, which will need lots of testing
but I'm hoping this will give people an idea of where I'm
going with this.

Jeremy.