* Replace the old parents in the block header with BlockLevelParents.
* Begin fixing compilation errors.
* Implement database serialization for block level parents.
* Implement p2p serialization for block level parents.
* Implement rpc serialization for block level parents.
* Add DirectParents() to the block header interface.
* Use DirectParents() instead of Parents() in some places.
* Revert test_block_builder.go.
* Add block level parents to hash serialization.
* Use the zero hash for genesis finality points.
* Fix failing tests.
* Fix a variable name.
* Update headerEstimatedSerializedSize.
* Add comments in blocklevelparents.go.
* Fix the rpc-stability stability test.
* Change the field number for `parents` fields in p2p.proto and rpc.proto.
* Remove MsgBlockHeader::NumParentBlocks.