CBG-4454: Add Database Init and Online fatal error stats

[bbrks]

CBG-4454

Add new stats for errors that cause databases to fail to come online (both with Init and StartOnline).
The stats are marked Internal, and are not committed for external or customer use - at least until we have a better understanding of their use-case.

• I would like a review from App Services team to actually make sure these stats are useful given the other changes to API for error reporting. I am unsure what the expectation of action is from these stats. Do they monitor and then use the API to find out more information?

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/000/