Instead of the direct shared_ptr.

Changes needed:
Moved loading of zones in config into the memory_datasrc implementation; at this moment the general API has no method for this (and it may not be necessary; datasources should handle their own config anyway).
This does have one drawback; through the dynamic loading mechanism we currently lose exception type information. So for now specific exceptions such as MasterLoadError end up as 'general' DataSourceErrors. Tests have been updated to reflect this.

Also, I made config handler slightly more lenient; it has hardcoded defaults and will now accept config parts without "class" or "zones"; using .spec -derived values is impractical here with the current config API, but in this case it shouldn't *really* matter; as IN is hardcoded in several places anyway atm.