Support replacing partitions of date-partitioned tables

This issue is about adding support to write to partitions of BigQuery's date-partitioned tables. Currently, reading is supported but inserts fails during execution of GbqConnector.delete_and_recreate: partition is deleted but creation fails due to existing table.

PR would include some refactoring to the current to_gbq function (e.g. getting rid of duplicate calls to BQ API).

Please see mremes/pandas-gbq for current changes. Currently not ready for a PR as I've encountered one issue with the BigQuery API (with subsequent create, delete, insertAll calls, insertAll's changes are not seen but after long waiting time).

[chalmerlowe]

This issue has seen no activity since 2017. Closing as will not fix.