flow.log: Add message IDs and filtering by message ID.

[ygoldfeld]

Filed by @ygoldfeld pre-open-source:

This had been on the informal flow.log road map already, but certain services' log volume / resource use (processor cycles, I/O, storage) raised the importance of this. Certainly there were other ways of dealing with it, and we did that, but at the time it highlighted the benefit of this hypothetically being implemented.

A detailed motivation and design are attached. Please read that carefully before implementing.
flow.log id and filtering design sketch.pdf