Sanity check loaded periodics and cap runtime.

BUG: 24900376
We've seen an issue where periodic run times can be massively
inflated after loading from disk.
As a safeguard, cap the period's loaded runtime to be [p, 2p]
from the time of loading.

Change-Id: Ie4464490c8d6702fee8efe9190c3da5dc5f013f6