Revert "Revert "varnish: restart backends every 3.5 days""
We are experiencing further instabilities w/ 7 days restarts:
https://bit.ly/2viW0Dx. Restart backends every 3.5 days.
This reverts commit 36dd660913e7cb2cb7680053fb15a5488fbc6495.
Change-Id: I6feda4d4c8bb704bd6840ef3ed5d0247adfa338d