RFC relay revamp, fourth installment

Here's the final installment of the relay cleanup patchset.

1-2 make the write path completely replaceable.
3 adds flags along with some related cleanup.
4-8 remove the padding in several stages.
9 simplifies the callbacks
10 removes the concept of sub-buffers

The new patches in this set are:

11 - remove the vmap of relay buffers

I think I've basically accomplished what I wanted to with this patchset
- getting rid of everything that was unnecessary and unused and fixing a couple of small design mistakes has made both the interface and the code much cleaner, closer to what it should have been from the beginning. The fact that it took less than a week of part-time hacking I think shows that the problems were mostly superficial, and the patches themselves pretty much reflect that - something in hindsight that should have been done long ago.

I don't plan on doing any more with this - if anyone else wants to, feel free. In the meantime, until the unified trace buffer replaces it, the current relay AFAIK works just fine and performs well for its existing users, so actually making these or any other changes at this point would I think be a wasted effort.

Tom