Refactor thrust::unique to use cub::DeviceSelect::Unique

[elstehle]

This is a sub-task of Thrust/CUB kernel consolidation #26

Refactor thrust::unique to use cub::DeviceSelect::Unique:

• Make thrust::unique use cub::DeviceSelect::Unique (see 948817e)
• Add dynamic 32/64-bit offset type-dispatch to thrust::unique (see 948817e)
• Add sanity tests for large number of items for thrust::unique (see 01f32dd)
• Add more elaborate testing for cub::DeviceSelect::Unique (see Adds tests for large number of items for cub::DeviceSelect #1612)