Confirm draft-ietf-ppm-dap-14 interop with Janus

[cjpatton]

Update Janus' interop runner to use the latest version of Daphne and fix any bugs that we encounter. One known issue is that the interop test API needs to be updated for DAP-14 (not yet published as of this writing).

Requirements:

1. We only need to support DAP-14 for Janus, so the internal_add_task API need not be backwards compatible with DAP-09. In particular, we can use the latest version VDAF config parameters. Note: our end-to-end tests use the internal_add_task API as well.
2. We should publish the docker image in the interop/ folder on dockerhub so that Janus can automate integration testing.