node/config: All time related config fields are in nanoseconds

[Wondertan]

The default values for GracePeriod and RoutingTableRefreshPeriod are set as time.Minute, however, in the config it is written in nanoseconds which is not very human-friendly.

Instead, it should be a string that is parsed via time.ParseDuration

[Wondertan]

Tentatively assigning @vgonkivs

[HoytRen]

It seems this is done? in the config.toml, I see 'GracePeriod = "1m0s"', and I believe it's spf13/viper handling the duration string.

[derrandz]

Yeah I suppose this is done cc @Wondertan