Also amend api_net.js to use returned length.

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 - 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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Rationale: this is the optimisation to make JS FFI-ed API
work nicer. We don't have strucuture introspection now,
and do not generate offsets automatially. Thus, using a
stable well-known location of the parsed message body is
better, in case of number of HTTP headers macro changes.

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Remove the specialized URI parser, clean up code a bit.

Fix parsing of URIs with IPv6 hosts (http://[2001:2:3::4]:567/)

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Frees up a bit of ram on ESP

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`mg_mqtt_match_topic_expression()` and friends are public now.

So far, only `#` is supported.

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For both OpenSSL and mbedTLS

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Optional specification of SSL/TLS cipher suites.

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Also fixed a memory leak on reconnect: previously, each reconnect was
creating a `struct mg_connection` which was never reclaimed

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Currently only OpenSSL API impl, but the plan is to add more.

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