paho-mqtt-cpp: upgrade 1.5.3 -> 1.6.0

Changelog:
  https://github.com/eclipse-paho/paho.mqtt.cpp/releases/tag/v1.6.0

Changes:
- Bumped Paho C submodule to v1.3.16 and updated directory name to externals/paho.mqtt.c
- Some significant performance increases (lower latency) for connect and publish
- Fixed topic_matcher and topic_filter to properly match parent with multi-level ('#') wildcard.
- Slight optimization of topic_filter to do simple string comparison if the filter does not contain wildcards.
- Set a minimum version for Paho C in the CMake file. Report the version found.
- .deb version properly set, and add architecture name to .deb file
- remove const from connect_options_builder 'move' constructor
- fix potential deadlock in thread_queue on capacity increase.
- Incorrect default retain value in a will options constructor
- prevent undefined behaviour on empty topic matching
- Sync reconnect example crashes on first reconnect

Signed-off-by: Andrej Kozemcak <andrej.kozemcak@siemens.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

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