Fix mountpoint dir removal on cancelled coroutine teardown

[touilleMan]

partially fix #715

[codecov]

Codecov Report

Merging #737 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #737      +/-   ##
==========================================
+ Coverage   86.51%   86.53%   +0.01%     
==========================================
  Files         159      159              
  Lines       11183    11184       +1     
==========================================
+ Hits         9675     9678       +3     
+ Misses       1508     1506       -2

Impacted Files	Coverage Δ
parsec/core/mountpoint/fuse_runner.py	97.11% <100%> (+0.02%)	⬆️
parsec/core/fs/userfs/userfs.py	85.61% <0%> (-0.47%)	⬇️
parsec/monitoring.py	62.85% <0%> (+11.42%)	⬆️

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