MINOR: hlua: use accessors for stream hlua ctx

MINOR: hlua: use accessors for stream hlua ctx

Change hlua_stream_ctx_prepare() prototype so that it now returns the
proper hlua ctx on success instead of returning a boolean.

Add hlua_stream_ctx_get() to retrieve hlua ctx out of a given stream.

This way we may easily change the storage mechanism for hlua stream in
the future without extensive code changes.

No backport needed unless a commit depends on it.