Fixed bugs:
- Priority queue lack of a `len + 1` logic in `in/2`
Changed behaviors:
- Topics not found in priority table (from config) will be treated with default priority,
instead of hasing topic name to a priority number.
- Default priority is now configurable (it was always lower than all configured priorities)
- The dropped message due to reaching `max_len` is now returned from `in/2`,
so the queue owner (`in/2` caller) can perform autopsy on it
Add request & response support for CONNECT & CONNACK
Prior to this change, there is no validate and specified process for
Request-Response-Information and Response-Information
Also added basic Request/Response functionality to emqx_client implementation
Use `mnesia:foldl` to traverse mnesia rather than `mnesia:first` and
`mnesia:next`, as a badarg exception would occur if the record was
deleted while travering the whole table.
turtleDeng