Close cesanta/dev#6974

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This can happen if the underlying LWIP TCP PCB has been destroyed (we got tcp_error_cb).

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By sending close_notify

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Frees up a bit of ram on ESP

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Flags should be 0 for all of these, except PUBREL (flags = 2).

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Message ID comes after topic, not before.

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Release pbuf sooner: after copying data from, before invoking user code

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E.g. given the expression `foo/#`:

- `foo/bar` matches
- `foo/` should not match
- `foo` should not match

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`mg_mqtt_match_topic_expression()` and friends are public now.

So far, only `#` is supported.

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