Remove limit on recurrent, add throttling

There are now two configurable settings that allow control over
scheduled recurrent runs.  One controls the maximum number of tasks that
can execute in parallel as part of a scheduled run:

    MAX_PARALLEL_PER_RUN : default 10

And another controls the largest number of "catch-up" runs a single
recurrent report can spawn as part of a scheduled run:

    MAX_INSTANCES_PER_RECURRENT_RUN : default 100

The related puppet work is submitted to operations-puppet-wikimetrics:

    https://gerrit.wikimedia.org/r/144154

Bug: 66841
Bug: 67543
Change-Id: Icddd5f725040b627d0b112c05d7c147acdc7641c