Checkpoint sync updates:
- Invalidate uncheckpointed forks to allow clients to switch to the checkpointed chain, previously clients would become stuck.
- Verify headers against current checkpoint.
- Add test for checkpoint sync.
Many updates to tests to run a complete set of feathercoind, feathercoin-qt and Python functional tests.
Upstream changes from Bitcoin:
https://bitcoin.org/en/release/v0.19.0.1
https://bitcoin.org/en/release/v0.19.1