- Move the accumulator definitions to a history accumulator file
- Add accumulator build helper calls + temporary database
- Add a header gossip content key encoding test
- Refactor & some cleanup
* Change History content key to us SSZ Union and adjust tests
* Change slot to byteBE instead of LE
This is currently not specified in the Portal network
specifications, but we are using already BE for the actual content
key, so change it also here to remain consistent.
Kim De Mey