drbd: Four new configuration settings for resync speed control

To reasonably control resync speed over drbd-proxy connections,
drbd has to measure the current delay of packets transmitted over
the (possibly congested) data socket vs the meta-data socket.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>