The Bio::DB::HTS object and other relatedHTS objects will be created
only once within a single HTSAdaptor. They will have only one copy and
be stored in $self->{_cache}->{_htsobj_handle}. Every use of HTS objects
should through this copy.

Also remove "close" calls to the HTS objects as these objects will
handle their resources properly when exiting. The DESTROY methods to
these objects has been added in
https://github.com/Ensembl/Bio-DB-HTS/pull/50
which will release the resources where needed.

This will provide a uniform interface to HTS objects, making it more
consistent. It prevents from creating multiple objects for the same
file and hence more resouces efficient too.

Also rename some variables to make them consistent throughout.

Related to ENSEMBL-5035.