1) Remove the whole ah rxbuf and put things on to the wsi buflist

This eliminates the whole detachability thing based on ah rxbuf
state... ah can always be detached.

2) Remove h2 scratch and put it on the wsi buflist

3) Remove preamble_rx and use the wsi buflist

This was used in the case adopted sockets had already been read.


Basically there are now only three forced service scenarios

 - something in buflist (and not in state LRS_DEFERRING_ACTION)

 - tls layer has buffered rx

 - extension has buffered rx

This is a net removal of around 400 lines of special-casing.