Bug 875562 - Part 7: Implement limits for stored crash data; r=Yoric

This patch institutes limits for how much crash data can be stored in
terms of maximum number of crash events per day. If more events than the
limit are encountered, we record that a high water mark has been
encountered and we silently discard the payload of future events.

We chose to not increment the version of the store payload because no
clients are actively saving this data yet, so it doesn't make sense to
incur a version bump.

--HG--
extra : rebase_source : 3d5d92668b38e8b9c3c45e1a61a3a172b576e0a5
extra : amend_source : 71fffa0724450c5fe13563d2539813e9a831b3de