Review retry functionality

There are three different ways in which we currently support retrying of tasks:

- `Retriable.Retry`
- `RetriableTaskRunner`
- `RetryTaskFactory`

We should review which of these are necessary and remove those which aren't.

There is also some code in the `RetriableTaskRunner` which firstly blocks the thread when sleeping and then uses `RunSynchronously` to block until the task is complete. If the decision is made that this code should not be removed, we should review the implementation.

Start by comparing the three retry entry points named in the issue: Retriable.Retry, RetriableTaskRunner, and RetryTaskFactory, and trace how each is used. Determine which APIs are necessary and whether RetriableTaskRunner should remain; if it does, review its blocking sleep and RunSynchronously behavior. Done means the unnecessary paths are removed or the remaining implementation is clearly reviewed and updated.

Written by the indexing model from the issue text.