* 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
* Add a basic ContentDB for Portal networks
* Use ContentDB in StateNetwork
* Avoid probably some form of sandwich problem by re-exporting kvstore_sqlite3 from content_db
* Allow for passing Portal specific bootstrap nodes
* Fix to also replaceNode when decodeMessage fails
* Add portal bootstrap node tests and reorder test cases