More effective compression for integer64 type

[MarcusKlik]

The integer64 type uses a double vector at it's base. Currently, this vector is compressed using the same compression schemes ar for double type columns. But knowing the type is really an 64-bit integer allows for more optimized compression, such as using a bit-shifter before compression (e.g. packing all 512_n-th_bits from a 32 kB block together and store them with an ordering along n).