Improve performance of path_to_filesystem()

[flesueur]

When using radicale with a large set of accounts, the PROPFIND request at /, even with simple owners_only access rights, is very long (more than 100 seconds with 100% CPU on my test).

The bottleneck seems to be in the function path_to_filesystem() which is called for every folder (so for every account, when listing /), more precisely in the part in which it lists again this large folder.

This patch aims to check the same cases (short names on windows and case-insensitivity) with a different approach. In my test, the PROPFIND at / goes from 120 seconds to 3 seconds.

[pbiering]

thank you very much

[pbiering]

flake8 is not happy:

flake8: commands[0]> flake8 .
./radicale/pathutils.py:285:9: F841 local variable 'safe_path_parent' is assigned to but never used
./radicale/pathutils.py:290:11: E128 continuation line under-indented for visual indent

-> please improve your PR, see also https://github.com/Kozea/Radicale/wiki/Development-Testing-Release

[MatthewHana]

Haven't tested it yet but thanks for this @flesueur. I was running into this bottleneck when I was debugging another issue and was going to have a look into the performance degradation later- glad that now I don't have to.

[pbiering]

flake8 is not happy:

flake8: commands[0]> flake8 . ./radicale/pathutils.py:285:9: F841 local variable 'safe_path_parent' is assigned to but never used ./radicale/pathutils.py:290:11: E128 continuation line under-indented for visual indent

-> please improve your PR, see also https://github.com/Kozea/Radicale/wiki/Development-Testing-Release

have fixed this now inline

[flesueur]

Sorry I was offline during this weekend. Thanks for having cleaned this PR !