Indexer: Added page and meta value rename functions

With these functions that search index can be updated after page moves or mass metadata updates without the need to reindex the whole page/wiki. These functions will be used by the [new pagemove](https://github.com/michitux/DokuWiki-Pagemove-Plugin) plugin. The functions are used for updating the index on page moves in order to make sure that the search index is always correct and doesn't grow during moves. Without updating the index it is very difficult (or even impossible) to find links after moving a couple of pages.

If one ignores the possibility that the indexer class might be replaceable one day it is possible to implement this as a separate indexer class that inherits from the indexer class, but this causes problems with the pid cache as after a page rename the pid cache returns an invalid id for the old page.

I'll add test cases for these new functions if the pull request is accepted in general.