unify /volume and /event-aggregates into /analytics

[Ivshti]

merge the /volume functionality into /event-aggregates, however:

• supports both cases: either authenticate as someone and get their stats only, or get global stats; however, global stats would be limited to certain combinations of periods and intervals (e.g. daily for a month, hourly for a day, etc.)
• the authenticated calls should get data either for a single channel, or for all channels; the global calls will always be for all channels
• rather than using the $day, $month and etc. mongodb operators, use what we use in volume ($mod): https://github.com/AdExNetwork/adex-validator/blob/05-stable/routes/volume.js#L29 ; each datapoint will use time as it's identifier, and should return it in a number format (rather than stringified date) ; this should greatly simplify the pipeline
• to avoid excessive CPU load, calls without authentication, that retrieve global stats, should be cached in redis for interval/2 time
• should be all handled in a separate file, routes/analytics.js; might use the channels middleware if we need to load a full channel

for example, aggr would be [{ time: 1563645559247, value: "1023022" }]

API:

• /analytics - optional authentication; queries for all channels
• /:channelId/analytics - required authentication; channel specific queries

If authentication is passed, it will only return aggregates for this particular earner; otherwise it will return a sum of all earners (like /volume now)

Both take timeframe, limit, eventType, metric; only certain combinations of timeframe and limit will be allowed for /analytics without authentication

Since this is a breaking change, it should be discussed with @ivopaunov and @samparsky first