Optionalbandwidth_Optionalcongestion_Optionalproposed_Proposed trie split for dynamic resharding
None: field missing (ShardChunkHeaderInnerV4 or earlier)
Some(None): field present, but not set (ChunkHeaderInnerV5 or later)
Some(Some(split)): field present and set
Optionalrent_TODO(2271): deprecated.
Optionalvalidator_TODO(2271): deprecated.
Contains main info about the chunk.