Fixes https://github.com/emqx/emqx-lua-hook/issues/118
1) If the Lua hook returns an empty list, it should replace the final
topic filter list.
2) The subscribed topics after running the 'client.subscribe' hook
should not contain topics filtered out by the hooks.
emqx_moduels for enterprise is refactored as a lib/plugin
in order to avoid merge conflicts, we make sure they can
co-exist in the same branch, because lib-opensource is compiled
in opensource project and lib-enterprise (to be added) is
compiled in enterprise project
And move emqx app back to root level
After realising the challenges of developing plugins in standalone
rebar projects. it is perhaps more smooth to keep using emqx as
a rebar dependency
Zaiming (Stone) Shi