We now let color_data keep its own state for
unused_colors, so that we longer have to pass in
a large list of unused_colors every time we want
to assign a new stream color.
This mostly matters at startup, where we might
be cycling through 5000 streams. We claim all
the unused colors up front.
Each operation now has an upper bound of expensiveness,
where the worst case scenario is basically popping
off the first element of a list of <= 24 colors.
The algorithm is now deterministic, too, to make
it easier to test. It's unclear whether random color
assignment ever had much benefit, and it made unit
testing the algorithm difficult. Now we have 100%
line coverage.
Fixes part of #10902.
