refactor needless context with wsi paramater passing

Now we bit the bullet and gave each wsi an lws_context *, many
internal apis that take both a context and wsi parameter only
need the wsi.

Also simplify parser code by making a temp var for
allocated_headers * instead of the longwinded
dereference chain everywhere.

Signed-off-by: Andy Green <andy.green@linaro.org>