[META 605] Add span links for message tracing

[felixbarny]

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Meta issue:

• https://github.com/elastic/apm/issues/605

Summary of spec changes:

• If your agent instruments any of Azure Service Bus, JMS, Kafka, RabbitMQ, SQS, or SNS, then instrumentation for receiving messages or batches of messages "SHOULD":
  • Create a transaction (with trace-context from message metadata, if any) for the handling of each message, if possible.
  • Otherwise, when creating the transaction or span for the message or batch of messages, look for trace-context in metadata for up to 1000 messages and create a span link for each.
• If your agent supports AWS Lambda instrumentation, then in addition to the creation of span links, this spec change has a number of changes to the context captured for SQS and SNS-triggered invocations. (The special-casing of a message batch with a single message has been dropped.)

[AlexanderWert]

Need to add handling of distributed tracing / span links for SQS and SNS for AWS Lambda instrumentation