MEDIUM: lua: move the cosocket storage outside of appctx.ctx

MEDIUM: lua: move the cosocket storage outside of appctx.ctx

The Lua cosockets were using appctx.ctx.hlua_cosocket. Let's move this
to a local definition of "hlua_csk_ctx" in hlua.c, which is allocated
from the appctx by hlua_socket_new(). There's a notable change which is
that, while previously the xref link with the peer was established with
the appctx, it's now in the hlua_csk_ctx. This one must then hold a
pointer to the appctx. The code was adjusted accordingly, and now that
part of the code doesn't use the appctx.ctx anymore.