* mqtt: expose "connfail" callback via :on()
This makes it just like all the other callbacks in the module and is a
revision of behavior called out in
https://github.com/nodemcu/nodemcu-firmware/pull/2967
* mqtt: clarify when puback callback fires
* mqtt: Don't reference stack buffers from the heap
The confusingly-named "mqtt_connection_t" object is just a triple of
- a serialized mqtt message pointer and length
- a buffer pointer (to which the above can be written)
- a message identifier
The last of these must be passed around the mqtt state machine, but the
first two are very local and the buffer is always sourced from the C
stack. Unfortunately, because the entire structure is persisted in the
heap, some callers assume that they can always use the structure without
reinitialization (see mqtt_socket_close), which will trash the C stack.
Sever the pairing between message id and local state, punt the local
state entirely out of the heap, and rename things to be less confusing.
30f706fb03