mod_dash: combine the rate vector and rate-adaptation techniques

Now we always use rate vectors, but we also use the rate-adaptation
to reduce the measured download speed when the download time exceeds
the two seconds threshold.

Discussed with and OK Antonio.
mod_dash/client_negotiate.py: add more rates

Discussed with and OK Antonio.
mod_dash: Tidy up some comments
mod_dash/client_negotiate.py: grammar fix
Merge remote-tracking branch 'origin/feature_dash_always_use_rates'