Refactor links to support properly backlinks in a farm

Current implementation of links is optimized for using a single wiki: the refactoring operation does not support fixing backlinks from other wiki without harmful performance issues.
In order to fix that we should refactor how links and backlinks are managed in order to support the farm of wiki.
A first proposal has been made in that direction in https://xwiki.markmail.org/message/7s2ondchhhgomuum to create a dedicated backlinks table as part of this refactoring to help managing them in all wikis.