6.3.0-rc only: ReadOnlySpan overwritten when multiple messages received before queue is processed

[Korkonius]

After the fetch thread is done enqueueing a message locally for dispatch through ReadOnlySpan it immediately moves on in most situations and frequently overwriting the referenced span.

If the message is then processed on a separate thread the client has already overwritten the old message so the referenced span has changed. This causes frequent de-serialization issues and exceptions thrown due to invalid JSON when parsing queued spans.