Add an 'on_disconnect_do' argument to

Add an 'on_disconnect_do' argument to
DBIx::Class::Storage::DBI::connect_info that, on disconnection, do what
'on_connect_do' does on connection.  Currently, this only works if the
code explicitly calls disconnect() on the Storage object.

While I'm here, make both 'on_connect_do' and 'on_disconnect_do' accept
code references as well as strings containing SQL statements.

Finally, remove code to call compose_connection() from DBICTest.pm that
never gets called any more.