* 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.
We just don't have the memory to be a real TLS client on the 8266. Put
that in a big box and point at it from the http and mqtt modules; others
may also wish to give reference.
* mqtt:connect() secure parameter should be boolean
Continue to honor the old 0/1 values, but make them undocumented and add
a deprecation warning to the code and docs. Eventually, this should go
away.
* mqtt: rip out deprecated autoreconnect
* mqtt: expose all the callbacks via :on