Allow Genesis block to start at epoch=N

[vincenthz]

Allow the system to start at a defined offset epoch=N instead of assuming epoch=0, which need to de-hardcode couple of place that expect to start at date=0.0

[edolstra]

Hm, what's the use case for starting at a non-zero epoch?

[edolstra]

I guess continuity with an existing chain might be one.

[vincenthz]

yes @edolstra, that's the reason