fix(aws-serverless): End the function.aws span when a callback-style handler calls back

[4ktLuffy]

For callback-style handlers the function.aws span ended as soon as the handler returned, so it had ~0 ms duration and ok status even when callback(err) came later. If the handler takes a callback and doesn't return a promise, the span is now ended by the wrapped callback instead. Tests call back from a setTimeout with a result and with an error.