fix(engine): Fix template rendering thread safety issue (#4828)
fix(engine): Fix template rendering thread safety issue (#4828)

Remove the engine `currentTemplates` field which was shared state
across threads and thus not thread safe, and instead just pass these
reference templates as parameters down recursively.

Closes #4819

Signed-off-by: Sean Eagan <4927d7efd5f318d1b28d811b65dee6a6ff5b62ca@att.com>
SlickNik/helm,jascott1/helm,SlickNik/helm,appscode/helm,jascott1/helm,appscode/helm