event aggregate API: query time periods, aggregate into time units

[Ivshti]

extension of #43

this would add an ability to query specific time periods via the API,

and furthermore merge many aggregate objects into aggregates for certain time units: e.g. "week", "day", "hour", "minute"

currently the AGGR_THROTTLE is set to 5 seconds, meaning that a "minute" unit would mean merging 120 aggregates into one

this should be done at a mongo level, otherwise it would be way too heavy