Remove dmlc::Timer and dmlc::optional

[hcho3]

Extracted from #285.

• Do not use dmlc/optional.h and dmlc/timer.h.
• Re-implement GetTime() and optional<>.

[hcho3]

This is needed to avoid build failure on MacOS.

[codecov]

Codecov Report

Merging #292 (45b6826) into mainline (262f782) will increase coverage by 0.01%.
The diff coverage is 89.65%.

@@              Coverage Diff               @@
##             mainline     #292      +/-   ##
==============================================
+ Coverage       84.99%   85.01%   +0.01%     
  Complexity         46       46              
==============================================
  Files             103      104       +1     
  Lines            7959     7979      +20     
  Branches           50       50              
==============================================
+ Hits             6765     6783      +18     
- Misses           1169     1171       +2     
  Partials           25       25              

Impacted Files	Coverage Δ
src/compiler/ast/ast.h	38.82% <0.00%> (ø)
src/predictor/thread_pool/thread_pool.h	96.22% <ø> (ø)
include/treelite/optional.h	88.23% <88.23%> (ø)
src/compiler/ast/fold_code.cc	86.79% <100.00%> (ø)
src/compiler/ast_native.cc	96.13% <100.00%> (ø)
src/predictor/predictor.cc	90.97% <100.00%> (+0.09%)	⬆️

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