Enable limited options from the client through to NDT server

[mattmathis]

Since BBR partially masks bufferbloat, it will enable new measurement opportunities if a client can request that NDT7 uses CUBIC congestion control, rather than BBR. We do need to carefully consider how this request should be carried and managed, and if the Locate service should participate in the conversation. This is an architectural level design consideration that may need to be generalized in the future to support additional options.

It is already the case that clients selects some service features by their protocol choices: NDT7 vs NDT5, SSL or not, which NDT5 variant, etc. In the future people may want choose TCP vs QUIC, and perhaps better controls over IPv4 vs IPv6.

Issues #350 already request that the actual CC be plumbed through to BQ, and #313 that we implement a holdback experiment, to do some CUBIC tests all the time.