Fix text extraction on DjVu so that it works beyond first call.

Fix text extraction on DjVu so that it works beyond first call.

This was caching the metadata tree in-process, but it was
returning this cache even for calls for things other then this
item.

As a result, if you did $file->getHandler()->getPageText( $file, 2 )
twice in a single request, the second time it would return false.

Change-Id: I6ee4dce3177c0898e98210d7471cfadc1108ca52