Add delay for processing if commit doesn’t exist

Created by: macraig

Some Sourcegraph instances (likely using a custom Git infrastructure to support monorepos) may receive a precise code intelligence upload before the remote known by Sourcegraph has been updated to include it.

We should support this case by allowing the uploader to specify a window in which the commit should become visible. Within this period, failure to resolve the target commit during processing should cause the processing job to be requeued.