The context manager `connection` always closes the connection

See merge request !26

The connection should be closed even if the abort raises an OperationalError.
An OperationalError can be raised with RelStorage for example when the
PostgreSQL server is restaring or is in recovery mode.

Assigning an unchanged value on primary indexed attribute does not raise an exception

See merge request !25

IndexedModelAttribute indexation

See merge request !24

Issue 54 model hooks

See merge request !23