perf: further speedup for 2.0 startup

[hanxiao]

2.0rc1 already achieves 2.2x speedup on startup and from jina import Document, Executor, Flow. This PR further improves the speed up by carefully inspecting

PYTHONPROFILEIMPORTTIME=x python -c 'from jina import Document, Flow, Executor'

and

PYTHONPROFILEIMPORTTIME=x jina -v

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This PR achieves 1.6x speedup on import (3.6x compare to 1.x):

➜  jina git:(perf-startup) ✗ hyperfine --warmup 1 'python -c "from jina import Document, Flow, Executor"'
Benchmark #1: python -c "from jina import Document, Flow, Executor"
  Time (mean ± σ):     350.4 ms ±   6.1 ms    [User: 265.7 ms, System: 398.7 ms]
  Range (min … max):   343.2 ms … 363.2 ms    10 runs

while master is at:

➜  jina git:(master) ✗ hyperfine --warmup 1 'python -c "from jina import Document, Flow, Executor"'
Benchmark #1: python -c "from jina import Document, Flow, Executor"
  Time (mean ± σ):     566.5 ms ±   7.3 ms    [User: 449.6 ms, System: 576.2 ms]
  Range (min … max):   556.3 ms … 582.8 ms    10 runs

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This PR achieves 1.7x speedup on CLI jina (3.4x compare to 1.x)

➜  jina git:(perf-startup) ✗ hyperfine --warmup 1 "jina -v"
Benchmark #1: jina -v
  Time (mean ± σ):     392.5 ms ±   3.6 ms    [User: 278.6 ms, System: 385.4 ms]
  Range (min … max):   386.3 ms … 397.0 ms    10 runs

while master is at:

➜  jina git:(master) ✗ hyperfine --warmup 1 "jina -v"
Benchmark #1: jina -v
  Time (mean ± σ):     689.2 ms ±  14.3 ms    [User: 613.7 ms, System: 903.1 ms]
  Range (min … max):   673.2 ms … 712.7 ms    10 runs

Optimizations:

• remove the heavy usage of pkg_resources (Importing pkg_resources is slow. Work is done at import time. pypa/setuptools#926, https://stackoverflow.com/a/9918496)
• carefully restructure/hide/remove head imports
• use https://github.com/ninjaaron/fast-entry_points to solve slow startup on Python 3.7.x (this is a bug in 3.7, 3.8+ does not require such patch)

[codecov]

Codecov Report

Merging #2510 (8a3130b) into master (cc87cd5) will decrease coverage by 4.24%.
The diff coverage is 76.74%.

@@            Coverage Diff             @@
##           master    #2510      +/-   ##
==========================================
- Coverage   83.29%   79.04%   -4.25%     
==========================================
  Files         152      152              
  Lines        9444     9417      -27     
==========================================
- Hits         7866     7444     -422     
- Misses       1578     1973     +395     

Flag	Coverage Δ
daemon	46.78% <56.97%> (-0.20%)	⬇️
jina	78.55% <76.92%> (-4.55%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
jina/helloworld/chatbot/app.py	0.00% <0.00%> (ø)
jina/helloworld/fork.py	0.00% <0.00%> (ø)
jina/jaml/helper.py	82.64% <ø> (-1.36%)	⬇️
jina/optimizers/__init__.py	0.00% <0.00%> (ø)
jina/optimizers/discovery.py	0.00% <0.00%> (ø)
jina/optimizers/flow_runner.py	0.00% <0.00%> (ø)
jina/parsers/helloworld.py	77.35% <ø> (-19.01%)	⬇️
jina/parsers/helper.py	35.76% <0.00%> (-6.63%)	⬇️
jina/__init__.py	70.31% <33.33%> (+0.61%)	⬆️
jina/helper.py	61.55% <37.50%> (-16.46%)	⬇️
... and 56 more

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