net/mwan3: add failure and recovery interval

net/mwan3: add failure and recovery interval

If mwan3track detects a failure the interval check will change to
"failure_interval" value until this interface is declared "offline".
After interface gets "offline" normal "interval" value is used again.
If interface ping is successful again "recovery_interval" value is used until
interface is declared "online" again.

Using the different intervals will accelerate the wan line detections on
failure and recovery.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>