[c-nsp] Link State Periodic SPF

[Adam Vitkovsky]

> ISIS standard (7.3.18) asks for at least one SPF every
maximumLSPGenerationInterval (which is, by default 15 mins

Interesting to see it's actually the full SPF run and in addition to that it
also does the FRR calculation at the same time. 
But still until I start seeing two digit number next to the calculation time
I don't need to worry about it I guess

adam