DRBD: worker

Our generic worker thread. Does the actual sending of data via the network
link. Does all the after-state-change activities, that have to be done
without holding the req_lock spinlock. And some other stuff.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>