Fix issue where startup of salt-master printed out "WARNING: Mixing fork() and threads detected; memory leaked."

c2e96e moved the init of various classes around. While doing so all of the module class init was moved into the parent process (pre-fork), which means that if any modules create threads we will leak memory (at least)
Merge branch 'develop' of github.com:saltstack/salt into develop
Merge pull request #18246 from jacksontj/develop

Fix issue where startup of salt-master leaks memory