Add Coroutine::join()

[matyhtf]

No description provided.

[codecov]

Codecov Report

Merging #4406 (c3f5089) into master (e648f7c) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4406      +/-   ##
==========================================
- Coverage   52.48%   52.47%   -0.02%     
==========================================
  Files          73       73              
  Lines       14809    14803       -6     
==========================================
- Hits         7773     7768       -5     
+ Misses       7036     7035       -1     

Impacted Files	Coverage Δ
src/reactor/base.cc	80.73% <ø> (-0.44%)	⬇️
src/coroutine/system.cc	32.03% <100.00%> (+0.07%)	⬆️
src/server/port.cc	50.00% <0.00%> (-0.28%)	⬇️
src/protocol/base.cc	63.15% <0.00%> (ø)
src/os/async_thread.cc	70.17% <0.00%> (+0.43%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e648f7c...c3f5089. Read the comment docs.