Transaction PDO table not upgradable

[judgej]

The PDO transaction model has createTable() to create the database table for storage of the registered transactions. The table is create from the Transaction metadata (all fields with a "store" flag set true).

When the metadata is extended in library releases, there is no updateTable() method that can be used to bring the table into line with the current metadata. It could with one.

[judgej]

The TransactionPdo model now includes an updateTable() method to do dirty blind ALTER TABLE statements. Call that to get the table into shape during any upgrades. It will set pdo_error_messages for any PDO errors found, but all should be "column already exists" type errors and so all can be ignored.

We may revisit this at a later date, if we want to do proper migrations that help it plug into other applications.