Fix mail jobs retrying setup properties, unify

Summary:

• removed retryUntil which was overriding $tries
• added backoff interval
• set tries/backoff to something more sensible and unified
• renamed one occurrence of old v7 property name into a proper one

Test Plan: ./phpunit

Reviewers: #hkccp_developers, vanmeeuwen

Reviewed By: #hkccp_developers, vanmeeuwen

Subscribers: #hkccp_developers

Differential Revision: https://git.kolab.org/D4424