core: default zero difficulty in genesis

[lightclient]

Right now we default to params.GenesisDifficulty when the difficulty is not configured for genesis. This causes the genesis to be a non-conforming "merged" block since the difficulty is non-zero. This PR will determine if a chain is merged (i.e. a timestamped fork is already active) and set the difficulty to zero in such instances. It retains the original behavior when the chain is not merged at genesis.

[MariusVanDerWijden]

Looks like there's one tracing test failing:

--- FAIL: TestSupplyOmittedFields (0.00s)
    supply_test.go:94: incorrect supply info:
        want {"blockNumber":0,"hash":"0x3055fc27d6b4a08eb07033a0d1ee755a4b2988086f28a6189eac1b507525eeb1","parentHash":"0x0000000000000000000000000000000000000000000000000000000000000000"}
        have {"blockNumber":0,"hash":"0x025ef71f6f4379c0001dbd64d34997cbdbdaabbad7b9a1346d7d02476e6438cc","parentHash":"0x0000000000000000000000000000000000000000000000000000000000000000"}
FAIL
FAIL	github.com/ethereum/go-ethereum/eth/tracers/internal/tracetest	0.388s

[fjl]

This is basically the same as my PR #31067 with a slightly different logic.

[lightclient]

closing in favor of #31067