Integrate Synthetic PoRep

[jennijuju]

For integrating https://github.com/filecoin-project/FIPs/blob/master/FIPS/fip-0059.md

we need to

Beta Give feedback

1. add network version 21, see #1726
2. add new seal proof, #1727

developing this in branch feat/sytheticporep until nv21 scope is finalized