Enable user to set which interface should be used for client network,
allowing client to be binded to the interface selected by user forcing
it to go through the selected interface.
Closes https://github.com/espressif/esp-mqtt/issues/253
The possibility to add a callback as custom handler was removed from
the client in favor of esp_event. These removes the older alternative
that wasn't possible to use.
Today there is no way to add a new transport without applying
modifications to the transport list. This impose limitations on the
client usage. Adding the custom configuration we enable user defined
transports.
The information was used only to log remaining messages on debug log.
It was checked on writing but updated prior to every call making the
verification meaningless.
