Validate that required fields are present when parsing a span

[LakerBaker]

We have clients who have sent spans to Haystack without an operation name, and the downstream services did not handle that well. In general micro-services should validate their inputs; haystack-collector needs to be updated to require operation name. Other validations are also possible; an email to the team that was not sending operation name read, in part, "these are the mandatory fields in a span: traceId, spanId, serviceName, operationName, startTime and duration."

[LakerBaker]

Fixed as documented; closing.

[LakerBaker]

There were some failing unit tests which were fixed in #45.