Investigate using `EpochNanoseconds` and implement wherever possible

[nekevss]

In #116, the EpochNanoseconds new type was added. The introduction of EpochNanoseconds was fairly bare, as further mentioned in #115. We should look into using EpochNanoseconds more throughout the codebase where it makes sense according to the proposal.

[JohannesHelleve]

I would like to try this one!