There are 2 issues with the code convention check ('make check_convention')
that runs as part of the build:
1. The 'pep8' CLI tool is deprecated, and 'pycodestyle' should be used instead.
2. There are some PEP8 errors.

This commit changes the Makefile to use 'pycodestyle' instead of 'pep8',
and fixes the remaining pep8 errors.

This reverts commit fba6b7c.
("Divide purge process to threads")

The reverted commit is faulty and breaks purge functionality.

…ile, not the link)