Trace into Task.async & similar spawned processes

[cschneid]

This has come up a few times, most recently trying to trace Absinthe (graphql) using Tasks.

Things to think about:

• UI to represent parallel vs. serial work?
• What to do about tasks that outlive the caller? (fire & forget)
• Tasks that are killed / stopped / crash?
• Processes spawned manually (not via Task.async apis)
• Background work done in GenServers (async & sync)