Consider Hardcoding Genesis Values as Config Parameters

[rauljordan]

In Prysm, we tend to use the blockchain service or other places in order to retrieve the chain's genesis time. However, this is not relevant for most networks that have already had a genesis. Instead, we can add the actual chain genesis times for the different networks we support under our params.BeaconConfig() which could be modified if a user wishes to via a custom configuration. This would eliminate a lot of boilerplate that checks for the chain genesis time and makes it easier for anywhere in the codebase to access than we do today.

[kasey]

I am assuming that going forward we will always use pre-generated genesis states for test networks. Given that assumption, we'll always have a genesis timestamp in the genesis state, which we can retrieve in the BeaconNode service and plumb through to the other services. I would prefer handling the timestamp that way over adding more values to config params.