Tunable genesis

[djrtwo]

Supersedes #1861 with final CI patches

--------- Original Text ---------------

fixes #1849 (by implementing option 2)

• renames MIN_GENESIS_DELAY to GENESIS_DELAY as this is a fixed constant now
• bumps GENESIS_DELAY to 172800 (2 days)
• simplifies genesis_time to be just eth1_timestamp + GENESIS_DELAY

[protolambda]

One thing I always liked, and part of the reason why the slot time is 12, and not 10 or 16, is that it aligns nicely with minutes, hours etc. With an arbitrary genesis time, and option 2, that breaks down to only aligning, but not being offset nicely.

So I've a slight preference for rounding (up) genesis time to the nearest hour. Or maybe the nearest minute.

Otherwise I agree with this change, it doesn't have to be offset exactly at midnight UTC.

[protolambda]

The alignment to the exact hour mark, or whatever offset we want to have for slot<>time conversion, is nice, but not critical. Once the genesis time is known, converting is really easy. We'll see if we miss the old behavior of perfect alignment to minutes/hours/days. For now though, this is all we need, so I am going to merge this.