TorClientBuilder: Add the ability to retry when fslock fails

With this branch, TorClientBuilder can now retry, up to a time limit, when it sees an error of kind LocalResourceAlreadyInUse.

Because of this change, it's now desirable to have an async version of create_unbootstrapped; I've added that too.

Since this is a top-level API, I'm hoping @gabi-250 or @Diziet will have a look at it before we merge.

Closes #1464 (closed).