api: tscache for tags

[flyingmutant]

Right now all /api/metric-tag-values queries go directly to the DB, which is clearly suboptimal.

Also, the cache may allow us to load all tag values together with time series (maybe?), and we'll be able to e.g. show total number of different values in each selector, without opening them (#147).

[flyingmutant]

Another idea for speed-up: maybe first return partial data (based only on first part of data), and in the background load the full thing and send it after. Does ch-go support this kind of output from a single query?

[flyingmutant]

Probably can have a single tscache variant for all merge-only data: uniques, percentiles, tag values.