mod_dash: negotiate rates and/or auto-adapt the rate

Rationale:

- once the test is out, we may want to tweak the rates without
  having to make a new Neubot release;

- in such case, we can do that by changing the code that runs
  on the M-Lab servers only;

- it may also be interesting to auto-adapt the rate, so add this
  possibility as an option;

- in particular, we auto-adapt if the server returns an empty
  rate vector in the negotiation phase.

I tested the auto-adapt at home for a long time, and the impact
on my quality of experience was reasonable.
js: add to-fixed to the LISP-like language
test/dash.json: fixup for what we show to the user