Require leaf statistics when expanding tree

[RAMitchell]

This PR extends the RegTree::ExpandNode() function to require summary statistics for the node being expanded as well as leaf weights for the left and right child. Calculating the leaf weights in advance for the child nodes required cacheing the left and right gradient sums from split calculation, so I have extended the SplitEntry class with this information.

There are several impacts to this change:

• Expanding a tree node is now a safe operation. After you call ExpandNode() the tree is complete and may be used immediately with all the required statistics and leaf nodes configured.
• As a consequence of the above, all code in tree updater algorithms that manually accesses RegTree internals such as tree nodes, statistics or leaf nodes becomes unnecessary (e.g. this). I have not remove these in this PR as it would make it too complicated, but I have verified that it is possible to remove them safely.
• Because the gradient sums in the newly created node children are cached, we no longer need to recalculate this. This means we can remove loops such as this and this for free. Again I will leave this for a future PR.

So in summary this will collapse a lot of the complexity around configuring a decision tree and will also improve performance. This is the way I have been doing things already in the GPU algorithms so it also helps me unify the code base.

[codecov-io]

Codecov Report

Merging #4015 into master will decrease coverage by 0.12%.
The diff coverage is 25.31%.

@@             Coverage Diff              @@
##             master    #4015      +/-   ##
============================================
- Coverage     57.31%   57.19%   -0.13%     
  Complexity      210      210              
============================================
  Files           190      190              
  Lines         15035    15065      +30     
  Branches        527      527              
============================================
- Hits           8617     8616       -1     
- Misses         6161     6192      +31     
  Partials        257      257

Impacted Files	Coverage Δ	Complexity Δ
src/tree/updater_histmaker.cc	2.87% <0%> (-0.04%)	0 <0> (ø)
src/tree/updater_colmaker.cc	1.55% <0%> (-0.05%)	0 <0> (ø)
src/tree/updater_skmaker.cc	2.17% <0%> (-0.08%)	0 <0> (ø)
tests/cpp/tree/test_prune.cc	100% <100%> (ø)	0 <0> (ø)	⬇️
tests/cpp/tree/test_refresh.cc	100% <100%> (ø)	0 <0> (ø)	⬇️
include/xgboost/tree_model.h	93.71% <100%> (+0.1%)	0 <0> (ø)	⬇️
tests/cpp/tree/test_param.cc	100% <100%> (ø)	0 <0> (ø)	⬇️
src/tree/updater_quantile_hist.cc	34.64% <33.33%> (-0.1%)	0 <0> (ø)
src/tree/param.h	86.43% <66.66%> (-0.08%)	0 <0> (ø)
... and 1 more

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[RAMitchell]

Ping @tqchen @hcho3

[hcho3]

LGTM. I left a comment. Can you look at it? @RAMitchell