Fix crashing stat mod never getting rescheduled

1.3.1 updated the cache to fetch stats in the background rather than
on demand. A new bug was added. If the stat mod crashes during
production of stats, it is never rescheduled.

Fix by rescheduling when crash is detected. Exponentially backoff
the schedule after an error so as not to spam the log.
Fix crash when vnode Pid vanishes during stat calc
Merge pull request #336 from basho/gh335-reshed-stats

Fix crashing stat mod never getting rescheduled