made ui auto refresh visual settings and stop always polling them

We're using our usual trick to publish url with ?v=<hash> appended at
the end. And we wake up all ui long polling requests if
internal_visual_settings change. When UI sees updated url it'll
automatically refresh visual settings.

New API is purely internal.

Change-Id: I0b669adc01da196a8725e606f75676a06d00d4ec
Reviewed-on: http://review.couchbase.org/35216
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>