Move more TorClient functions to InertTorClient

InertTorClient was introduced in !2370 (merged) to enable the keymgr CLI to use TorClient functionality without accessing the network or the on-disk cache dir (for context, see #1496 (closed)).

TorClient wraps an InertTorClient, which the docs say is "Used for accessing the key manager and other persistent state". We should:

• rethink InertTorClient's role in TorClient. InertTorClient advertises itself simply as "A Tor client that is not runnable" ("An InertTorClient never connects to the network." ). So, in TorClient, we should update inert_client's docs to say that its purpose is more broad (i.e. instead of saying that the inert client is just for accessing the keystores and other persistent state, we could say it offers client functionality that doesn't require network access)
• review the TorClient API to see if there any other functions that should be moved to InertTorClient (such as create_onion_service)