Enable venvs.reset() concurrent execution

[gogoduan]

• change the internal API name of worker: send_action -> send, get_result -> recv (align with envpool)
• add a timing test for venvs.reset() to make sure the concurrent execution
• change venvs.reset() logic

[codecov-commenter]

Codecov Report

Merging #517 (d3e2d15) into master (cd7654b) will decrease coverage by 0.04%.
The diff coverage is 90.24%.

❗ Current head d3e2d15 differs from pull request most recent head fd8eee9. Consider uploading reports for the commit fd8eee9 to get more accurate results

@@            Coverage Diff             @@
##           master     #517      +/-   ##
==========================================
- Coverage   94.41%   94.37%   -0.05%     
==========================================
  Files          63       63              
  Lines        4246     4249       +3     
==========================================
+ Hits         4009     4010       +1     
- Misses        237      239       +2     

Flag	Coverage Δ
unittests	94.37% <90.24%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
tianshou/env/worker/base.py	79.06% <75.00%> (+2.32%)	⬆️
tianshou/env/worker/dummy.py	89.28% <75.00%> (-3.03%)	⬇️
tianshou/env/worker/ray.py	85.36% <83.33%> (-1.82%)	⬇️
tianshou/env/venvs.py	91.61% <100.00%> (ø)
tianshou/env/worker/subproc.py	94.07% <100.00%> (-0.04%)	⬇️
tianshou/policy/modelfree/npg.py	97.70% <0.00%> (-1.15%)	⬇️

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