BUG: Fix error when reading postgres table with timezone #7139

Fixes an issue where read_sql_table() will throw an error if it is
reading a postgres table with `timestamp with time zone` fields that
contain entries with different time zones (such as for DST).

This also adds a new keyword, convert_dates_to_utc, which optionally
allows the caller to convert `timestamp with time zone` datetimes into
UTC, which allows pandas to store them internally as
`numpy.datetime64`s, allowing for more efficient storage and operation
speed on these columns.