Code review comment for lp:~vanvugt/mir/simplify-BufferQueue

The compositor rendering more frames than a client is producing is in fact a common occurrence as most apps are mostly idle. So in that case the growth of the current_buffer_users vector is unbounded. Regardless of any performance benefit to the average case, the worst case for a vector is clearly unacceptable (bug 1317370). We need to use a container that guarantees uniqueness of its elements.