netbuf: Use 'cursor' semantics regions with start_flush

This fixes an issue where sometimes the same data would be flushed twice
because a call to start_flush() was made before a prior call to
end_flush had completed. For some reason this was only reproducible on
IOCP/windows.

Change-Id: I466c56ca4e927f4b881a604817c994aa7766095c
Reviewed-on: http://review.couchbase.org/38328
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>