Instead of splicing up a cloned copy of stream_assignment_colors
every time somebody uses a color, we just rebuild a hash
of used_colors from our subscribed streams when we need to assign
a color, and we avoid calling into stream_color.pick_color() when
a stream already has a color.
This change has a slight functional impact in the situation where
a user unsubscribes some streams during their session, because
we weren't "reclaiming" colors before on unsubscription, but the
simple approach gets that for free.
(imported from commit adf360365bdf1ae9db99c533a0bde62d91f5dfe8)
7d67716c21