put the QUIC layer in control of selecting the path (mode 3)

[marten-seemann]

Currently the draft says:

Upon the completion of path validation, the QUIC stack passes the result (successful or failed) back to the ICE stack. The ICE stack then nominates an address pair. The client SHOULD then migrate the QUIC connection to this path in a timely manner.

As pointed out by @martinthomson on the list, we should put the QUIC layer in charge of selecting a path when using mode 3. A QUIC stack (multipath or not) will necessarily have some path selection / scheduling logic. We don't need to hand off the decision to ICE.