Default the network key will be taken from a network key file
instead of randomly generated on each run. This is done because
the data that gets stored in the content database is dependant on
the network key used, as the node id is derived from this.
- 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