- Move keep alive management up to the Mongoose library
 - Only outgoing control messgaes should reset keepalive timer
 - Add unit test

https://forum.mongoose-os.com/discussion/1155/mqtt-keep-alive-compliance-issue

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`mg_mqtt_match_topic_expression()` and friends are public now.

So far, only `#` is supported.

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It would be probably good idea to also remove tools/docgen.py and
asciidoc.mk, but asciidoc.mk is still mentioned under `cloud/doc`, which
may contain some useful info which we'll need at least to review before
removing.

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Until I read the doc and find how to limit the retention,
otherwise it just eats all my ram and cpu and things start to fall
apart.

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Comments in headers are changed a bit: removed adoc-specific stuff,
markdown is used instead

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