Extract interaction constraint from split evaluator.

[trivialfis]

• Extract interaction constraints from split evaluator.

The reason for doing so is mostly for model IO, where num_feature and interaction_constraints are copied in split evaluator. Also interaction constraint by itself is a feature selector, acting like column sampler and it's inefficient to bury it deep in the evaluator chain. Lastly removing one another copied parameter is a win.

• Enable inc for approx tree method.

As now the implementation is spited up from evaluator class, it's also enabled for approx method.

• Removing obsoleted code in colmaker.

They are never documented nor actually used in real world. Also there isn't a single test for those code blocks.

• Unifying the types used for row and column.

As the size of input dataset is marching to billion, incorrect use of int is subject to overflow, also singed integer overflow is undefined behaviour. This PR starts the procedure for unifying used index type to unsigned integers. There's optimization that can utilize this undefined behaviour, but after some testings I don't see the optimization is beneficial to XGBoost.

Related to #4732 .

[RAMitchell]

Looks good! This is quite a nice feature upgrade for the 'histmaker' algorithm.

[trivialfis]

@hcho3 @RAMitchell I enforced the row index to be uint64_t as size_t is different for 32 bit system and 64 bit system. This might change memory usage on 32 bit system, does that seem to be a reasonable change?

[trivialfis]

Note:
On OSX:

typedef unsigned int         uint32_t;
typedef unsigned long long   uint64_t;
typedef unsigned long       __darwin_size_t;

[codecov-io]

Codecov Report

Merging #5034 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5034   +/-   ##
=======================================
  Coverage   71.52%   71.52%           
=======================================
  Files          11       11           
  Lines        2311     2311           
=======================================
  Hits         1653     1653           
  Misses        658      658

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