Make unspecified periodic spaced tasks run on default interval

When there are multiple tasks with a defined spacing, they can get
somewhat staggered and force the periodic task processor to run
additional iterations. Since tasks with no spacing run every time the
periodic task processor runs, they get run more frequently than one
would expect.

Some may argue that this change in behavior could cause some task to
behave differently than it used to. However, there was never any
guarantee that the task would run more often than every 60 seconds, and
in many cases the tasks may already run less frequently than that (due
to other long-running tasks). So this change should not introduce any
new issues related to the timing of task execution; it would only serve
to make the timing more regular.

We should also make the default interval configurable but that's
reserved for a separate change.

DocImpact: periodic tasks without a specific spacing interval will now
run on the default interval (currently every 60 seconds) rather than
whenever the task processor runs.

UpgradeImpact: see above; periodic tasks without a specific spacing
interval will now run on the default interval which may be less
often than some tasks run now, but with this change they will run
on a more consistent interval.

Closes-Bug: #1319232
Related-Bug: #1276203
Related-Bug: #1272830

Change-Id: I4dd20f68175165d311fa4d3f459c86f2dcac911b
Merge "Make unspecified periodic spaced tasks run on default interval"