Fix: cache issues

[saikonen]

• Adds a check that tmpdir exists when launching workers, and creates the dir if it is missing. This fixes workers not being able to launch if for any reason the cache directory gets wiped.
• Adds log cache process to the cache healthcheck monitoring, so it will correctly be restarted in case of issues.

[ruial]

This fix is very much needed, the LogCacheStore tends to die after a while and the service requires a manual restart.

Could someone please review? @oavdeev @obgibson @jackie-ob