added docs and tests for mqtt library, small fixes (removed warnings, option for custom outbox, websocket bug fixed for longer transports). refactored to use common tcp_transport component, support for CMake build system.
Closes#2108
The CI uses the mirror-submodule-update.sh for non-permanent reassignment
of 3rdparty sub-modules to local mirrors (only for 'non-master' branches).
List of the mirrors in mirror-list.txt
You can use the script locally to speed up data retrieval on network problems
check_submodule_sync job uses 'GIT_STRATEGY: clone' to check the availability of public sources