Split chainparams by network

Summary:
See title.

Depends on D8289 and D8297 and D8306

Test Plan:
ninja check
Run mainnet and testnet nodes. Updated ABC and BCHN chains using these nodes.

bitcoind -daemon
bitcoind -daemon -testnet
./contrib/source-control-tools/patch-recipes/update-chainparams.sh abc 8332 18332 bchn 8332 18332
git show   # verify chainparams updated as expected

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8298