Search backend: move concurrency handling to caller of LogBackendEvent

Created by: camdencheek

Previously, LogBackendEvent spun up its own goroutine, and used the passed-in wait group to handle cleaning it up. It's generally preferable to leave the responsibility of spinning up goroutines to the caller, and this is complicating some refactors I've got queued, so this modifies it to be a synchronous function that can instead be backgrounded by the caller.

Test plan

Unit tests and manual smoke tests.