Remove direct uses of PageIdentifier objects coming from Confluence-XML logs

[raphj]

To fix this, find all usages of PageIdentifier in the Confluence Migrator Pro code and fix them.

We can't really use PageIdentifier objects coming from logs because when we upgrade Confluence-XML, the class is unloaded and then reloaded, but this breaks a lot of stuff for existing object, including instanceof PageIdentifier, which becomes false.

At best, using the date from a serialization of the object (for instance by calling toString() on it and ensuring this serialization is stable) would probably be fine, but remember that we can't use instanceof PageIdentifier to identify objects from the logs. Getting the class name and comparing it with a string should work. If we can help it, we should do it another way.

This is the reason for this change, that avoids using PageIdentifier when we can use a Long representing the page Id instead: 1b2ff4a#diff-e3b1eba238490cd712f73f1b6b88e1efd2acfd898ae95ee9f02cd2b41fa256f2R208-R212 (Sorry I didn't do it in a separate commit, I was in a rush)

[raphj]

See https://extensions.xwiki.org/xwiki/bin/view/Extension/Extension%20Module/Extensions/JAR/