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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We don't have a real Array.prototype of course, just `[1,2,3].length`
behaves like we do.

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Must be invoked on mongoose main task, not on LWIP

Also request immediate poll oif there is data to be sent and space to
send it.

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For both OpenSSL and mbedTLS

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It's confusing

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cesanta/mongoose-os#186

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...and fix a leak where UDP connections with SEND_AND_CLOSE set would not be closed

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Close cesanta/mongoose#770

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And also fix `flash` target for nrf51

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And set to 1024 for mOS.

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Close cesanta/dev#6485

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Relate to cesanta/mongoose#780

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This reverts commit 1a17e17c462bdd4e1d26d8742f8b7087273e04c2.

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