avoid possible metadata subscription data loss

Previous implementation append filer logs into one file. So one file is not always sorted, which can lead to miss reading some entries, especially when different filers have different write throughput.

weed/filer/filer.go

@@ -44,6 +44,7 @@ type Filer struct {
 	Signature           int32
 	FilerConf           *FilerConf
 	RemoteStorage       *FilerRemoteStorage
+	UniqueFileId        uint32
 }
 
 func NewFiler(masters []pb.ServerAddress, grpcDialOption grpc.DialOption,
@@ -54,6 +55,7 @@ func NewFiler(masters []pb.ServerAddress, grpcDialOption grpc.DialOption,
 		GrpcDialOption:      grpcDialOption,
 		FilerConf:           NewFilerConf(),
 		RemoteStorage:       NewFilerRemoteStorage(),
+		UniqueFileId:        uint32(util.RandomInt32()),
 	}
 	f.LocalMetaLogBuffer = log_buffer.NewLogBuffer("local", LogFlushInterval, f.logFlushFunc, notifyFn)
 	f.metaLogCollection = collection