drbd: New net configuration option socket-check-timeout

In setups involving a DRBD-proxy and connections that experience a lot of
buffer-bloat it might be necessary to set ping-timeout to an
unusual high value. By default DRBD uses the same value to wait if a newly
established TCP-connection is stable. Since the DRBD-proxy is usually located
in the same data center such a long wait time may hinder DRBD's connect process.

In such setups socket-check-timeout should be set to
at least to the round trip time between DRBD and DRBD-proxy. I.e. in most
cases to 1.

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