BUG: Fix error when reading postgres table with timezone #7139
Closes #7139, merged via https://github.com/pydata/pandas/commit/c3eeb577b2b06ecc65e96c5f5de69f0fba01c3dd

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.
