server: cleanup span download function

[dt]

• Move the tasks into their own free functions for readability and future testability.
• Use GoAndWait to avoid mistakes around what runs in/out of group.
• Use a nested group instead of counting channel sends for worker termination check.
• Add commentary throughout.

Release note: none.
Epic: none.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is 

[dt]

This is one where hide whitespace makes the side-by-side diff a good deal cleaner.

[msbutler]

Nice!

[dt]

TFTR!

bors r+

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)