Fix beforeTimestamp comparison for event log timestamps
Needed to do the same timezone-stripping logic to the timestamp argument before performing
the SQL comparision.
Also, Renamed beforeTimestamp => beforeTimestampMillis, changed the type from float to the str
representation of an int (to get around 32-bit integer limitations), did the milliseconds => seconds
conversion on the python side.
Test Plan: bk
Reviewers: dgibson, dish
Reviewed By: dish
Differential Revision: https://dagster.phacility.com/D7724