CPU optimizations - 'hist' method

[SmirnovEgorRu]

Moved discussion from PR #5008 to the issue. After the PR - performance drop on CPU is observed. I prepared a plan how to establish optimizations from PR #4529 in master branch again.

I tried to split large changes to separate PRs:

№	Description	Status
1	Extend Performance Timer to track overall performance of each algorithmic stage for whole training	Merged
2	Add benchmarks to track performance changes (~3 data sets are enough for the start, it can be extended later)	Merged
3	Add nested parallelism in EvaluateSplit function + tests	Merged
4	Add nested parallelism in BuildHist function + tests	Merged
5	Add nested parallelism in ApplySplit function + tests	Merged

Expected timelines for creation the all PRs - EOY.

[trivialfis]

I would love to see the performance optimization back and sorry for the oversight. Feel free to ping me if you need any help.

[hcho3]

Add benchmarks to track performance changes (~3 data sets are enough for the start, it can be extended later)

For now, a script that we can run is okay, so that performance results can be re-produced. I plan to design and build an automatic system for performance benchmarking.

[SmirnovEgorRu]

All PRs are merged, final results:

Dataset	higgs1m	Letters	Airline-ohe	MNIST	MSRank-30K	Mortgage
Before commit #5008, sec	15.5	10.3	55.3	69.5	99.1	18.3
After commit #5008, sec	123.5	144.6	133.2	Inf	332.2	53.4
Current Master, sec	14.7	10.1	59.4	80.5	111.6	19.0

Current master is is slightly slower on MNIST data set than it was before, but it can be covered in the next PRs

Thank you all who helped me to refactor the optimizations to have them in the master again (@hcho3, @trivialfis, @RAMitchell ).