Refcount-based fix for index memory releasing problem

Created by: jmarshall

On #67 (closed), I wrote

It would probably be possible to add a reference to the Bio::DB::HTSfile to the Bio::DB::HTS::Index object to get the objects destroyed in the right order. Either in the XS code (non-trivial) or in the Perl wrappers (hopefully easier).

This implements that. First attempt was via sv_magic(), but that didn't get the ordering right. Using an external global hash is a bit more manual but also more explicit.