CL: Docs nits

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 * Change return type of mg_mgr_poll to return number of events
 * Add mg_mgr_min_timer
 * Refactor main poll loop to remove LwIP-specific stuff

CL: Refactor mg polling

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CL: Add `mg_url_encode_opt()` - a parametrized version of `mg_url_encode()`

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Which is a replacement of (deprecated) `mg_http_parse_header`, but,
similarly to `asprintf`, allocates a new buffer if the client-provided
one is not large enough.

Also use it throughout mongoose code, and thus some header-related
limitations are removed; in particular,
https://github.com/cesanta/mongoose/issues/813 is fixed.

CL: Mongoose Web Server: Deprecate `mg_http_parse_header()` and implement `mg_http_parse_header2()` instead, which allocates a new buffer if the client-provided one is not large enough (similarly to `asprintf`).
CL: Mongoose Web Server: Fix limitations of header value lengths, e.g. when parsing authentication headers such as nonce, etc.

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CL: none

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CL: Mongoose Web Server: Remove tunneling functionality which was used by nobody

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CL: Mongoose Web Server: Digest authentication: Fix nonce validity check (expired nonce or nonce from the future did not cause the the check to fail)
CL: Mongoose Web Server: Digest authentication: Fix nonce request value; it worked before because nonce validity check was broken as well
CL: Mongoose Web Server: Digest authentication: Add `nonce` argument to `mg_http_create_digest_auth_header()`: clients should use the value received from the server's authentication request.

Resolves https://github.com/cesanta/mongoose/issues/809

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CL: Mongoose Web Server: Websocket: Respond to Ping with Pong
CL: Mongoose Web Server: Websocket: Properly close a connection with Close frame (in response to a client's close and when protocol failure is detected)
CL: Mongoose Web Server: Websocket: Fix support of fragmented messages
CL: Mongoose Web Server: Websocket: Add support for control frames interjected in the middle of a fragmented message

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Change the API to heap-allocate the string and get rid of a bunch of fat stack buffers.

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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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