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'
mod_dash/client_smpl.py: print more info at LOG_INFO level
mod_dash/client_smpl.py: move the check on the max iter number

Move the check on the max iter number, so when we print the
informative message on the current run we also include the
last run.
mod_dash/client_smpl.py: raise the max. number of iterations

I have experimentally noticed that 15 iterations lead to a more
or less 20 seconds tests, while 20 iterations are closer to 30
seconds (which is the duration we expect for this test).
mod_dash: also save request ticks

Bump the server and client schema because we added a new
field to the saved fiels.

Manually verified that the new piece of data is saved.

Requested by Antonio.
Merge branch 'feature_dash_last_tweaks'
Neubot/0.4.16.4 released