Add fork block checkpoints and update defaultAssumedValid and min chain work (main and testnet)

Summary:
Changes to chain params:

• added fork block checkpoints for mainnet and testnet3
• update defaultAssumeValid for both chains
• updated nMinimumChainWork to match the defaultAssumeValid blocks

Having a check point on fork block means nodes cannot be led onto wrong
UAHF-compatible chain.

Knowing that you are on right Bitcoin Cash chain after the fork block
can be used to optimize some code which determines whether we are pre-fork
or post-fork.

Test Plan:
make check
Run bitcoind with the changes on mainnet and testnet

Reviewers: #bitcoin_abc, deadalnix, sickpig

Reviewed By: #bitcoin_abc, deadalnix, sickpig

Subscribers: sickpig

Differential Revision: https://reviews.bitcoinabc.org/D480