* Reduce Nim 1.6 compiler warnings/hints for Fluffy and Nimbus proxy
Mostly raises Defect removals, TaintedString removal and some
unnecessary imports.
Also updating the copyright years alongside.
* Further reduce Nim 1.6 compiler warnings/hints for Nimbus
- 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
- Allow access to contentDB from portal wire protocol
- Use this to do the db.get in `handleFindContent` directly
- Use this to check the `contentKeys` list in `handleOffer`
* Add SSZ Unions through case objects
* Add connection id content response test and improve other test vectors
* Implement content keys and ids for state network as per spec
Content keys case object is used so that it can be serialized and
deserialized as an SSZ Union.
* Let message Union in Portal wire protocol start at 0 as per new spec
Kim De Mey