batcher refactor: flatten out go routines

[geoknee]

This is a refactor to try and make the two goroutines spawned by the driver equally clear.

• Names the previously anonymous receiptsLoop
• Adds some godoc comments about what each loop does
• Tries to be a little more idiomatic about passing contexts in to functions

[semgrep-app]

Semgrep found 1 err-todo finding:

• op-program/client/l2/engineapi/l2_engine_api.go
  • L500 - Triage

TODO in error handling code

_{Ignore this finding from err-todo.}