Complete cudf support. (#4850)

* Handles missing value.
* Accept all floating point and integer types.
* Move to cudf 9.0 API.
* Remove requirement on `null_count`.
* Arbitrary column types support.

tests/distributed/test_issue3402.py

@@ -1,5 +1,6 @@
 #!/usr/bin/python
 import xgboost as xgb
+import numpy as np
 
 xgb.rabit.init()
 
@@ -59,6 +60,7 @@ X = [
    4415.50,22731.62,1.00,55.00,0.00,499.94,22.00,0.58,67.00,0.21,341.72,16.00,0.00,965.07,
    17.00,138.41,0.00,0.00,1.00,0.14,1.00,0.02,0.35,1.69,369.00,1300.00,25.00,0.00,0.01,
    0.00,0.00,0.00,0.00,52.00,8.00]]
+X = np.array(X)
 y = [1, 0]
 
 dtrain = xgb.DMatrix(X, label=y)