Update checkpoints for mainnet

[barton2526]

We really need to update the checkpoints. Last checkpoint is:

Gridcoin-Research/src/checkpoints.cpp

Line 55 in 004c6b8

(1050000, uint256("0x0753b624cc0ab39d8745b436012ce53c087f7b2e077099e746a9557f569a80f3") )

Block 1,050,000 is from October of 2017 (almost exactly 1.5 years ago). We should add checkpoints for the Elizabeth (mandatory) release and I would advise making this a regular event for future mandatories to get in the habit of doing it regularly.