Extend further validation of config values (#8469)

- 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

cmd/routers.go

@@ -22,17 +22,6 @@ import (
 	"github.com/gorilla/mux"
 )
 
-func newObjectLayerFn() (layer ObjectLayer) {
-	globalObjLayerMutex.RLock()
-	layer = globalObjectAPI
-	globalObjLayerMutex.RUnlock()
-	return
-}
-
-func newCacheObjectsFn() CacheObjectLayer {
-	return globalCacheObjectAPI
-}
-
 // Composed function registering routers for only distributed XL setup.
 func registerDistXLRouters(router *mux.Router, endpoints EndpointList) {
 	// Register storage rpc router only if its a distributed setup.