Set default outcome according to spec

[axw]

Apart from HTTP and gRPC, which set outcome based on the status code, set the default outcome to "success" by default, or "failure" if any errors were captured within the transaction or span.

This brings us into line with the agent spec for transaction and span outcome:

• https://github.com/elastic/apm/blob/master/specs/agents/tracing-transactions.md#transaction-outcome
• https://github.com/elastic/apm/blob/master/specs/agents/tracing-spans.md#span-outcome

For other protocols, we can default to the following behavior:

• failure when an error is reported
• success otherwise

[apmmachine]

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[marclop]

Looks great, very cool use of read locks and locks

[marclop]

nice, this access doesn't need to be coordinated because the writer acquires a s.mu.Rlock and modifications won't be allowed to happen since this function holds s.mu.Lock

[axw]

Yup :) Maybe it would be worth commenting on the locks when they should be held. The SpanData/TransactionData locks only need to be held when holding an RLock on the Span/Transaction lock.