Agg timestamps should be consistent between WS/REST

[charlescc9]

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

[charlescc9]

Also, I realize this would be a breaking change, so maybe something for the 2.0 roadmap if you have one.