BUG/MAJOR: http: use a static storage for sample fetch context

Baptiste Assmann reported that the cook*() ACLs do not work anymore.

The reason is the way we store the hdr_ctx between subsequent calls
to smp_fetch_cookie() since commit 3740635b (1.5-dev10).

The smp->ctx.a[] storage holds up to 8 pointers. It is not meant for
generic storage. We used to store hdr_ctx in the ctx, but while it used
to just fit for smp_fetch_hdr(), it does not for smp_fetch_cookie()
since we stored it at offset 2.

The correct solution is to use this storage to store a pointer to the
current hdr_ctx struct which is statically allocated.