* Rename FindNode to FindNodes as per spec
* Use consistently lower case starting camelCase for consts
Style guide nep1 allows for both CamelCase and camelCase for
consts, but we seem to use more often camelCase. Using this now
consistently.
* Change some procs to func
* 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
