Previously, catalog tables were not fully protected from malicious
non-superusers. This PR fixes permission handling be severely
restricting permissions to the catalog and instead using SECURITY
DEFINER functions to alter the catalog when needed without giving
users permission to do those same operations outside of these functions.
In addition, these functions check for proper permissions themselves
so are safe to use.
This PR also makes sure that chunk tables have the same owner as the
hypertable and correctly handles `ALTER TABLE...OWNER TO` commands to
keep this info in sync.
Clean up the table schema to get rid of legacy tables and functionality
that makes it more difficult to provide an upgrade path.
Notable changes:
* Get rid of legacy tables and code
* Simplify directory structure for SQL code
* Simplify table hierarchy: remove root table and make chunk tables
* inherit directly from main table
* Change chunk table suffix from _data to _chunk
* Simplify schema usage: _timescaledb_internal for internal functions.
* _timescaledb_catalog for metadata tables.
* Remove postgres_fdw dependency
* Improve code comments in sql code
