This implements the Streaming Tail Worker API as discussed previously, should be a lot more elegant and less error-prone.
What isn't cleaned up yet:

• Commit history, splitting this into more digestible commits (possible to some extent)
• Many TODOs are cleared up, one is added about the Onset spanId.
• We have good tests for the spanId field defined for SpanOpen, its meaning has changed and it's now actually called spanId. We'd still want more coverage for the outer spanId defined in TailEvent, which is not included in tests as we only look at the inner event so that tests are deterministic (TailEvent includes timestamp) @mar-cf if you want to develop tests that should be helpful still, or I can add them here

Downstream PR to follow.