rename proto field

This should not have any impact.

weed/pb/volume_server.proto

@@ -478,7 +478,7 @@ message VolumeInfo {
     repeated RemoteFile files = 1;
     uint32 version = 2;
     string replication = 3;
-    uint32 BytesOffset = 4;
+    uint32 bytes_offset = 4;
     int64 dat_file_size = 5; // store the original dat file size
     uint64 expire_at_sec = 6; // expiration time of ec volume
     bool read_only = 7;