Agg timestamps should be consistent between WS/REST

Is your feature request related to a problem? Please describe.
Currently the Timestamps values for the REST models.Agg and WS models.EquityAgg structs are different in two ways:

1. Agg only has the start timestamp, EquityAgg has start and end
2. Agg timestamp is a time.Time, EquityAgg is an int64

I realize that the REST agg and WS agg aren't necessarily the same exact data (the WS agg contains more info in each datapoint), but I think it would be very useful (and hopefully a light lift) to standardize the timestamps for ease of client development.

Describe the solution you'd like

1. EquityAgg timestamps changed to time.Time (ideally all API timestamps would be time.Time)
2. Agg given an end timestamp OR remove EquityAgg end timestamp and add a Timespan field to both structs

Locate the models.Agg and models.EquityAgg entry points and compare their timestamp fields and usages in the REST and WebSocket clients. Resolve whether the API should use time.Time consistently or align both structs around an end timestamp or Timespan field; the work is done when the chosen representation is consistent across both models and their exposed API behavior.