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