Logging: truncate long byte fields

This is important for the like of `FetchBundle` where megabytes
of binary data can be shuffled around and could clog the logs
(and waste processing power to format).

It turns out that `MessageToString` is implemented in Python, making
it easy to override, although we have to rely on the `_Printer` class.
It would be better to propose UPSTREAM to protobuf.
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