Remove the use of dmlc::JSONWriter, dmlc::Stream, and dmlc serializer

[hcho3]

Extracted from #285.

• Do not use dmlc/json.h, dmlc/io.h, and dmlc/serializer.h.
• Replace all uses of dmlc::Stream with std::fstream and std::stringstream.
• Replace dmlc::JSONWriter with rapidjson::Writer.
• Use uint64_t for storing instance counts in JSON.
• Rename TaskParameter to TaskParam, to be consistent with ModelParam.
• Replace ReferenceSerialize with SerializeToJSON. Previously, ReferenceSerialize was a reference implementation of serializer for tree models. ReferenceSerialize was useful for verifying the correctness of other serializer methods of the tree objects. The reference serializer had to go, unfortunately, because it used dmlc serializer functions. As a replacement, I implemented a JSON serializer SerializeToJSON. While not very efficient compared to binary serializers, the JSON serializer will be useful for debugging and testing.
• Reimplement LoadText with std::istream and std::getline.

[codecov]

Codecov Report

Merging #289 (88cae4e) into mainline (f769f5b) will decrease coverage by 0.84%.
The diff coverage is 93.03%.

@@              Coverage Diff               @@
##             mainline     #289      +/-   ##
==============================================
- Coverage       84.77%   83.92%   -0.85%     
  Complexity         46       46              
==============================================
  Files             102      102              
  Lines            7821     7889      +68     
  Branches           50       50              
==============================================
- Hits             6630     6621       -9     
- Misses           1166     1243      +77     
  Partials           25       25              

Impacted Files	Coverage Δ
include/treelite/tree.h	97.77% <ø> (ø)
src/compiler/ast/ast.h	38.82% <ø> (ø)
src/compiler/ast/builder.h	100.00% <ø> (ø)
include/treelite/tree_impl.h	90.04% <50.00%> (-0.05%)	⬇️
src/frontend/xgboost.cc	80.08% <75.86%> (-0.94%)	⬇️
src/json_serializer.cc	92.17% <92.17%> (ø)
include/treelite/annotator.h	100.00% <100.00%> (ø)
src/annotator.cc	91.47% <100.00%> (+0.73%)	⬆️
src/c_api/c_api.cc	96.65% <100.00%> (ø)
src/compiler/ast/load_data_counts.cc	90.90% <100.00%> (ø)
... and 26 more

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[trivialfis]

Looked briefly. Not a complete review.