Removing usage of celery chains from report scheduling

Removing usage of chains had additional benefits like being able to use
CELERY_ALWAYS_EAGER on paralelilsm tests and being able to remove sleeps
and recursion limts.

Goes with corresponding puppet change: https://gerrit.wikimedia.org/r/153390
Tested rolling active editor on eswiki on dev.
Wrote some benchmarks on wiki below, please be aware that numbers vary greatly
what are important are orders of magnitude.
http://www.mediawiki.org/wiki/Analytics/Editor_Engagement_Vital_Signs/Backfilling

Bug: 68840
Change-Id: Ie4432d72a600450203395052e2bdfab348285882