net: mqtt: Add BSD socket implementation

Add new, socket based MQTT implementation, based on MQTT from Nordic
nRF5 SDK, introducing the following features:

* transport independent MQTT logic, with support for multiple transports
* support for multiple MQTT versions (3.1.0 and 3.1.1 supported)
* single event handler - no need to keep callback array in RAM
* automatic send of Ping Requests, for connection keep-alive
* message/event parameters wrapped into strucutres - easier extension
  for future MQTT versions
* no separate thread needed to run MQTT - application only needs to call
  mqtt_input and mqtt_live periodically

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>