Consider updating StreamReceiver to encrypt token

[sappenin]

The latest JS ilp-protocol-stream is no longer generating a random token, but is instead encrypting it based upon the server secret. We should consider doing the same.

See code here: https://github.com/interledgerjs/ilp-protocol-stream/blob/master/src/server.ts#L200

Also see note here describing the differences: https://github.com/interledgerjs/interledgerjs/blob/master/packages/pay/src/controllers/asset-details.ts#L73