Client: Don't panic on evict

Important for two reasons:

• Makes client eviction testable in replica_test.zig and the simulator.
• Allows (language) clients to reconnect with a new session if they so choose.

I agree with not panic-ing in general, but what would be the desired behaviour here? Evict the client session / not panic the thread / keep the thread open for retry / timeout the thread if retry is not attempted?

I would worry about a potential situation where too many threads are opened and not getting the resources harvested in time and creating a bad situation.

I'm not sure! It probably depends somewhat on the language in question, too.