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 plugin.
Add test cases for indexer rename page/meta value methods
Indexer rename test cases: assert that renames succeed
Clarified the behavior of the Doku_Indexer::renamePage method
Indexer: added internal deletePageNoLock method

The new deletePageNoLock method is used by renamePage and avoids that
the index needs to be unlocked and locked again for deleting the page.
Indexer: abort page rename if deletion of new id fails
Merge pull request #159 from michitux/indexer

Indexer: Added page and meta value rename functions