new lwbuf api and mpsafe sf_bufs

[Samuel J. Greear]

>    The sfbufs are statically allocated and thus have scaling issues
>    (e.g. the example Samuel gave on IRC is when one running many parallel
>    sendfile()'s).   I would like to see the SFBUF code use a more
>    dynamic model and a sysctl which sets the maximum number of *excess*
>    sfbufs instead of the maximum number of sfbufs.

I thought I would ask this before laying down any code, you (Dillon)
mentioned putting this into an rbtree previously as opposed to the
existing hash.  Even if we do a variable hash (maybe just one that
grows, not shrinks?) amortized costs should be lower than an rbtree.
Is there any rationale as to why an rbtree would be preferrable in
this case apart from simply being simpler in implementation compared
to a variable hash?

Sam