In #2907 we implemented a cache for ACME clients (accounts registry, here).

When we set up (create or update) a new ACME Issuer we add a client for it to the cache (here or here). This client is then used by challenge and order controllers. The client holds, between other, the ACME account key.
Currently we only remove the client in the cache in some scenarios, when setting up the ACME issuer fails here.

We may want to remove the client in other scenarios (i.e when Issuer becomes not ready due to failure to retrieve account key or when Issuer is deleted). For context, see this comment and description of #2907).

We may also want to not remove the client when it is not necessary, i.e the account key has not changed.

/kind cleanup