public api remove context from user callback API BREAK

Since struct lws (wsi) now has his own context pointer,
we were able to remove the need for passing context
almost everywhere in the apis.

In turn, that means there's no real use for context being
passed to every callback; in the rare cases context is
needed user code can get it with lws_get_ctx(wsi)

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