#11 fix int64 issues with VCF_VARIANT64 defined

[Hauke Duden]

.. also renames the pd(U)Long64 constants to pd(U)Int64

Note that there is a compiler error in
VariantData::toString because DateTimeSpan has no
constructor that takes an uint64. However, this error
must have been there with long64 as well.

[Hauke Duden]

[kdmult]

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Applied at r2977.