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/api-errors.go

@@ -263,6 +263,7 @@ const (
 	ErrInvalidResourceName
 	ErrInvalidLifecycleQueryParameter
 	ErrServerNotInitialized
+	ErrBucketMetadataNotInitialized
 	ErrRequestTimedout
 	ErrClientDisconnected
 	ErrTooManyRequests
@@ -1295,7 +1296,12 @@ var errorCodes = errorCodeMap{
 	},
 	ErrServerNotInitialized: {
 		Code:           "XMinioServerNotInitialized",
-		Description:    "Server not initialized, please try again.",
+		Description:    "Server not initialized yet, please try again.",
+		HTTPStatusCode: http.StatusServiceUnavailable,
+	},
+	ErrBucketMetadataNotInitialized: {
+		Code:           "XMinioBucketMetadataNotInitialized",
+		Description:    "Bucket metadata not initialized yet, please try again.",
 		HTTPStatusCode: http.StatusServiceUnavailable,
 	},
 	ErrMalformedJSON: {
@@ -2211,6 +2217,10 @@ func toAPIErrorCode(ctx context.Context, err error) (apiErr APIErrorCode) {
 		apiErr = ErrInvalidMaxParts
 	case ioutil.ErrOverread:
 		apiErr = ErrExcessData
+	case errServerNotInitialized:
+		apiErr = ErrServerNotInitialized
+	case errBucketMetadataNotInitialized:
+		apiErr = ErrBucketMetadataNotInitialized
 	}
 
 	// Compression errors