Precompute our fork ID and store it inside the discv4 process

[MegaRedHand]

We're going to the DB each time we receive an ENRResponse message. This is less than ideal, so we should instead precompute this and store it locally in the process, to make request/response handling as light as possible.

[lakshya-sky]

Hey @MegaRedHand, #5539 checks and updates forkid in local node record so we can just read it form the local node record in the disc server. Furthermore, after #5546 it will be even faster to read forkid from the local node record.