Don't let client slowness possibly block JVM threads

## Context

As discussed in https://github.com/clojure-emacs/cider-nrepl/pull/773#discussion_r1227086600, it seems possible that given a slow- or buggy-enough client, sending logs from the JVM would actually block the given JVM thread that is performing the logging.

i.e., clojure code like:

```clj
(log/error :foo)
```

might block if Emacs happens to have some sort of issue. Which doesn't seem incredibly rare given Emacs' customizability.

## Proposed solution

Make sure that the actual logging call happens from a thread executor. One communicates with said thread by message passing. One can check whether the thread is alive by using e.g. `deref` with its `[ref timeout-ms timeout-val]` extended signature.

And one could have a nrepl command for "resetting" the executor to a new one, which would help if it ever got stuck.