Move ParserSession internal to FullTextIndexer

Since there's nothing to configure for the ParserSession, we may as well
let the FullTextIndexer create a new one in its own constructor. This
*could* pose problems if more than one FullTextIndexer is running at a
time (as one ParserSession could close resources used by a different
FullTextIndexer), but currently the locking in FullTextThread precludes
this possibility.