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Performance improvement for `cache()` with PostgreSQL: use multirow VALUES syntax.

Minor updates:
- docs: updated Usage
- pgstorage: change from INTEGER to BIGINT. Fixes 12

pgstorage: rollback transaction in exception handlers

Fix for 10 psycopg2.errors.SyntaxError: each UNION query must have the same number of columns

Fix for 9 NameError: name 'InvalidAttr' is not defined

- added `remove_view` API to drop a view
- added `rename_view` API to rename a view
- added `get_view_data` API to get all (or a set) of view data, including names, types, and "appdata" blobs
- added new `UnknownViewname` exception
- map PostgreSQL/psycopg2 `UndefinedColumn` exception to `InvalidAttr`
- map PostgreSQL/psycopg2 `UndefinedTable` exception to `UnknownViewname`