do not panic on rebalance during server restarts (#19563)

This PR makes a feasible approach to handle all the scenarios
that we must face to avoid returning "panic."

Instead, we must return "errServerNotInitialized" when a
bucketMetadataSys.Get() is called, allowing the caller to
retry their operation and wait.

Bonus fix the way data-usage-cache stores the object.
Instead of storing usage-cache.bin with the bucket as
`.minio.sys/buckets`, the `buckets` must be relative
to the bucket `.minio.sys` as part of the object name.

Otherwise, there is no way to decommission entries at
`.minio.sys/buckets` and their final erasure set positions.

A bucket must never have a `/` in it. Adds code to read()
from existing data-usage.bin upon upgrade.

cmd/bucket-versioning-handler.go

@@ -82,7 +82,7 @@ func (api objectAPIHandlers) PutBucketVersioningHandler(w http.ResponseWriter, r
 		}, r.URL)
 		return
 	}
-	if _, err := getReplicationConfig(ctx, bucket); err == nil && v.Suspended() {
+	if rc, _ := getReplicationConfig(ctx, bucket); rc != nil && v.Suspended() {
 		writeErrorResponse(ctx, w, APIError{
 			Code:           "InvalidBucketState",
 			Description:    "A replication configuration is present on this bucket, bucket wide versioning cannot be suspended.",