Stream events instead of maintaining an in-memory copy in visualiser

[aryanjassal]

Specification

Currently, the tracer web visualiser loads the spans in memory and iterates over them multiple times to figure out the best way to format them in their individual swimlanes. We should never be loading the entire data in memory, so the visualiser needs to be updated to reflect this.

To test this, it can be run on a running polykey agent to view how the updates happen live.

Additional context

Tasks

1. Spans shouldn't be stored in memory
2. The spans should retain their swimlane selection logic
3. The visualiser should support live runs

[linear]

ENG-625