Remove unneeded temporary device vector for strings scatter specialization

[davidwendt]

The specialization logic for scatter on strings column includes building a temporary device_vector of string_view objects for the source column. The builtin iterators for an input column_view will work for string_view and so this extra device_vector is not required.

The utilities for creating a device_vector for string_views is also changed to use device_uvector instead. This also removed an unnecessary parameter as well as other minor changes.

I also added a gbenchmark for scatter that includes strings. Removing the extra device_vector showed a small 10-15% performance improvement.

[codecov]

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[kkraus14]

cmake lgtm

[harrism]

@gpucibot merge