refactor concurrent work limiting in sync in x/sync

[danlaine]

Why this should be merged

It's better to wait and then start a goroutine than to create a goroutine and then wait.

How this works

Instead of making a goroutine to handle the work item, then have the goroutine wait until we're under the concurrent work limit, don't create the goroutine until we're under the concurrent work limit.

How this was tested

Existing UT