This PR fixes the handling of drop_chunks when the hypertable's
time field is a TIMESTAMP or DATE field. Previously, such
hypertables needed drop_chunks to be given a timestamptz in UTC.
Now, drop_chunks can take a DATE or TIMESTAMP. Also, the INTERVAL
version of drop_chunks correctly handles these cases.
A consequence of this change is that drop_chunks cannot be called
on multiple tables (with table_name = NULL or schema_name = NULL)
if the tables have different time column types.