[Cake] Heaps of cake

[Jonathan Morton <chromatix99@gmail.com>]

I’ve just pushed my latest set of updates, the main feature of which is the replacement of the linear search in cake_drop() with a heap structure.  For efficiency, the heap is constructed the first time a packet drop is required (on overflow) and maintained incrementally thereafter until 64K packets have been delivered without an overflow occurring.

Please do hammer on these codepaths for all you’re worth.  It compiles and survives a smoke-test, that’s all I know so far.

I also took the opportunity to remove a variable which was widely read but never written, except for initialisation.  The number of queues per tin is thus now a compile-time constant.  This should be an efficiency improvement, as well as removing magic numbers from a few places.

 - Jonathan Morton