[generate_dump] Revert generate_dump optimization PR's sonic-net#2599"…

…, add fixes for empty /dump forder and symbolic links

- What I did
3442815 Revert "Revert "Optimize the execution time of the 'show techsupport' script to 5-10%, (sonic-net#2504)""
c3bd01f Revert "Revert "[generate_dump] Optimize the execution time of 'show techsupport' CLI by parallel function execution (sonic-net#2512)""

Add a fix for the empty /dump folder inside the final tar archive generated by the show techsupport CLI command.
Add a fix to not follow the symbolic links to avoid duplicate files inside the final tar archive generated by the show techsupport CLI command.

- How I did it
Modify the scripts/generate_dump script.

- How to verify it
1. Manual verification
do the show techsupport CLI command and save output original.tar.gz (with original generate_dump script)
do the show techsupport CLI command and save output fixes.tar.gz (with the generate_dump script modified by this PR)
unpack both archives original.tar.gz and fixes.tar.gz
compare both directories with ncdu & diff --brief --recursive original fixes Linux utilities
2. Run the community tests
sonic-mgmt/tests/show_techsupport

Signed-off-by: vadymhlushko-mlnx <vadymh@nvidia.com>