BUG Properly deprecate old 'memory' config setting

Recently the `$databaseConfig['memory']` option was removed, but there was missing a proper deprecation, meaning anyone using this in their code had it silently revert to using a file based connection.

This fix restores the original option, but with a deprecation message.

I assume that the target for this module is version 1.3, since 1.2 is SS 2.4 targetted.