Added finding quantiles on GPU. (#3393)

* Added finding quantiles on GPU.

- this includes datasets where weights are assigned to data rows
- as the quantiles found by the new algorithm are not the same
  as those found by the old one, test thresholds in
    tests/python-gpu/test_gpu_updaters.py have been adjusted.

* Adjustments and improved testing for finding quantiles on the GPU.

- added C++ tests for the DeviceSketch() function
- reduced one of the thresholds in test_gpu_updaters.py
- adjusted the cuts found by the find_cuts_k kernel

src/common/compressed_iterator.h

@@ -111,7 +111,7 @@ class CompressedBufferWriter {
     symbol <<= 7 - ibit_end % 8;
     for (ptrdiff_t ibyte = ibyte_end; ibyte >= (ptrdiff_t)ibyte_start; --ibyte) {
       dh::AtomicOrByte(reinterpret_cast<unsigned int*>(buffer + detail::kPadding),
-                   ibyte, symbol & 0xff);
+                       ibyte, symbol & 0xff);
       symbol >>= 8;
     }
   }