Since `gpb` is a GPL compile-time-only dependency, we should not
include it in the release.
Note that adding `gpb: :none` to the release applications list *will*
make Mix include its files *and* reference them in the startup +
release scripts...
In the current implement, mqtt-sn gateway is using the emqx_session as
its session structure. We need a method to get a pkt_id for sending like
REGISTER related messages