Convert cudf::merge to use device_uvector instead of device_vector

[harrism]

Converts cudf::merge to use device_uvector.

Contributes to #7287

Performance is improved:

Comparing /home/mharris/rapids/cudf/cpp/build/merge_before.json to /home/mharris/rapids/cudf/cpp/build/merge_after.json
Benchmark                                          Time             CPU      Time Old      Time New       CPU Old       CPU New
-------------------------------------------------------------------------------------------------------------------------------
Merge/pow2tables/2/manual_time                  -0.0831         -0.0791             0             0             0             0
Merge/pow2tables/4/manual_time                  -0.0924         -0.0930             1             1             1             1
Merge/pow2tables/8/manual_time                  -0.0813         -0.0808             4             3             4             3
Merge/pow2tables/16/manual_time                 -0.1172         -0.1170             9             8             9             8
Merge/pow2tables/32/manual_time                 -0.0790         -0.0790            19            18            20            18
Merge/pow2tables/64/manual_time                 -0.0757         -0.0757            46            43            46            43
Merge/pow2tables/128/manual_time                -0.0730         -0.0730           111           103           111           103

[codecov]

Codecov Report

Merging #8043 (e19e138) into branch-0.20 (51336df) will decrease coverage by 0.04%.
The diff coverage is 83.27%.

@@               Coverage Diff               @@
##           branch-0.20    #8043      +/-   ##
===============================================
- Coverage        82.88%   82.84%   -0.05%     
===============================================
  Files              103      103              
  Lines            17668    17804     +136     
===============================================
+ Hits             14645    14750     +105     
- Misses            3023     3054      +31     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/io/orc.py	86.89% <ø> (ø)
python/cudf/cudf/utils/cudautils.py	57.75% <25.00%> (ø)
python/cudf/cudf/utils/dtypes.py	81.87% <41.66%> (-1.57%)	⬇️
python/cudf/cudf/core/column/numerical.py	94.43% <72.72%> (ø)
python/cudf/cudf/core/tools/datetimes.py	80.25% <74.69%> (-4.28%)	⬇️
python/cudf/cudf/core/groupby/groupby.py	91.55% <76.92%> (+0.11%)	⬆️
python/cudf/cudf/core/column/decimal.py	90.83% <77.27%> (-2.09%)	⬇️
python/dask_cudf/dask_cudf/backends.py	89.51% <80.00%> (-0.08%)	⬇️
python/cudf/cudf/core/column/column.py	88.64% <83.33%> (ø)
... and 27 more

---

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[karthikeyann]

LGTM.

[karthikeyann]

<> preferred.

Suggested change

	#include "rmm/cuda_stream_view.hpp"
	#include <rmm/cuda_stream_view.hpp>

[rgsl888prabhu]

Looks good, have to update copyright

[harrism]

@karthikeyann @rgsl888prabhu please don't approve when changes are required, to prevent it getting merged before the fixes are made.

Addressed and need to get debug build fixes in urgently.

[harrism]

@gpucibot merge