NIFI-8380: Removed requirement in validation for working directory and extensions directory to exist; removed auto-creation of directories in validation
NIFI-8380: Fixed a few thrading bugs, so that if we have multiple threads trying to download/unpack extensions, we properly synchronize the unpacking and unpack into the correct sub-directory under the working directory
Signed-off-by: Pierre Villard <pierre.villard.fr@gmail.com>
This closes#4950.