drbd: Add drbd_thread->resource and make drbd_thread->connection optional

In the drbd_thread "infrastructure" functions, only use the resource instead of
the connection.  Make the connection field of drbd_thread optional.  This will
allow to introduce threads which are not associated with a connection.

Signed-off-by: Andreas Gruenbacher <agruen@linbit.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>