Convert all commit steps to async

In the work for #593 the commit steps that involved scanning were not converted to an async model. This caused #672 which created a seperate thread pool for these commit steps that could block. Ideally these commit steps would eventually be converted to an async model.

Start by reviewing the work in #593 and the separate thread-pool change in #672 to identify the scanning commit steps that remain synchronous. Trace those steps and define completion as converting all of them to the async model without a separate blocking thread pool.

Written by the indexing model from the issue text.