Initial message timeout support

Sometimes app message is sent&delivered,
but neither ackHandler nor nackHandler gets called.
For such cases, let's add a timeout.
js: send next message on timeout
js: call on_failure callback when message has timed out

This approach (sending next message on timeout)
should prevent situations when neither ackHandler nor nackHandler
was called, but message was sent (or not sent).
FIXME: if message was really not sent, what to do?..
Debug/logs
Merge branch 'appmessage-timeouts'
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