Remove dependence on color constants in ServerReadOnlyMemory.

This fixes a maintainance problem where you have to update this otherwise
unrelated file to keep it in sync whenever you add a color constant.

I've added a B_COLOR_WHICH_COUNT constant to the color_which enum which should
be updated to point to the newest color constants as new ones are added. I
reworked ServerReadOnlyMemory to use this constant instead of using to the
current largest color constant directly. If you use B_COLOR_WHICH_COUNT to
refer to a color in your code expect to get unpredictable and nonsensical
results. Most likely you'll get an undefined result which will return black
but don't depend on it.

The net effect of this is that ServerReadOnlyMemory doesn't need to be updated
anymore when new color constants are introduced but will continue to produce
correct results.

Eliminate kNumColors constant, replace it with B_COLOR_WHICH_COUNT