[Cake] Backporting 'tc class' support

[Jonathan Morton]

> On 16 Jul, 2018, at 1:41 am, Kevin Darbyshire-Bryant <kevin at darbyshire-bryant.me.uk> wrote:
> 
> AFAICT the tin order makes no difference whatsoever these days, indeed the dequeue mechanism picks up from the last tin and spins around rather than starting at either 0 or highest tin each time.

While it might not be entirely apparent from the code, the tin selector has two-phase behaviour, and this makes the tin ordering important.

If *any* tin contains traffic *and* meets its local schedule, the *highest* such tin is selected unconditionally.  Only if *no* such tin can be found does the search revert to finding the tin with traffic and with the most-nearly-satisfied schedule.  Both conditions are resolved on a single pass through the tin array.

In diffserv3/4, the Best Effort tin is set at the same rate as the global shaper.  Therefore, its local schedule is always met whenever the global schedule is.  Therefore, if it were placed higher in the tin list than Bulk, it would be able to strictly starve out Bulk traffic, which is not what I want.  Instead I give Bulk the next higher tin and a very restrictive schedule.

 - Jonathan Morton