Limit length of metadata decode debug output

Limit the size of the raw metadata that gets printed when debug logging is enabled.
1. You haven't documented anywhere this special handing of `max == 0`; (I personally would rather not treat 0 as a special case, and instead pass in `max: Option<uint>`, but reasonable people can differ on this sort of detail.) It might be a good idea to note the special case in a note above `DATA_TRUNCATE`.

2. More important: since every call to `data_log_string` is passing in `DATA_TRUNCATE`, and `data_log_string` is private to this module, why make it a parameter at all?  Why not just use `DATA_TRUNCATE` directly in `data_log_string` ?