Fixes an issue reported by @klauspost and @vadmeste
This PR also allows users to expand their clusters
from single node XL deployment to distributed mode.
- This PR allows config KVS to be validated properly
without being affected by ENV overrides, rejects
invalid values during set operation
- Expands unit tests and refactors the error handling
for notification targets, returns error instead of
ignoring targets for invalid KVS
- Does all the prep-work for implementing safe-mode
style operation for MinIO server, introduces a new
global variable to toggle safe mode based operations
NOTE: this PR itself doesn't provide safe mode operations
This is an effort to remove panic from the source.
Add a new call called CriticialIf, that calls LogIf and exits.
Replace panics with one of CriticalIf, FatalIf and a return of error.
