Factor out table sharing into a helper module

Issue Summary

In http://drupal.org/node/133812 I'm looking for ways to integrate og_sites with singlesignon. Maybe a good step would be to factor out the dynamic table sharing from og_sites into a new helper module that could be used by various multisite modules (og_sites, singlesignon, etc.).

This might look like a new module implementing a hook, 'sharedtables', that returns tables to be shared with a master/hub site. Different modules could return overlapping sets. Like og_sites, sharedtables would use hook_init() to do the sharing.