* Create BlockParentBuilder.
* Implement BuildParents.
* Explictly set level 0 blocks to be the same as direct parents.
* Add checkIndirectParents to validateBlockHeaderInContext.
* Fix test_block_builder.go and BlockLevelParents::Equal.
* Don't check indirect parents for blocks with trusted data.
* Handle pruned blocks when building block level parents.
* Fix bad deletions from unprocessedXxxParents.
* Fix merge errors.
* Fix bad pruning point parent replaces.
* Fix duplicates in newBlockLevelParents.
* Skip checkIndirectParents
* Get rid of staging constant IDs
* Fix BuildParents
* Fix tests
* Add comments
* Change order of directParentHashes
* Get rid of maybeAddDirectParentParents
* Add comments
* Add blockToReferences type
* Use ParentsAtLevel
Co-authored-by: stasatdaglabs <stas@daglabs.com>
* 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.
