Mark RubySingleThreadExecutor as a SerialExecutorService

[meineerde]

This resolves #1069.

However, I'm still not 100% sure if the RubySingleThreadExecutor is actually serialized. From my understanding of the code, it is, but there may be edge-cases I'm missing (such as when the thread dies).

As such, I'd still appreciate if someone who knows the (intended) RubyThreadPoolExecutor semantics to review this.