Intermediate commit.

Restructured UserColumn to use a bitfield for various flags rather than separate values.
Copy the bitfield around between ColumnMetadata and PEColumn.  Arrange for the bitfield
to contain the previously derived attributes for key part, unique key part, primary key part.
Use the bitfield in the migrated infoschema queries for show columns, columns info_schema table.

Todo:
- An upgrade
- Multitenant mode
- show full columns
- migrate the tests that load the catalog directly (i.e. via metadata file) to
  instead load through the front door.