feat(nats-connection): implement named executor thread factories

[krzysiekigry]

goodbye, "pool-2-thread-1"

[scottf]

@krzysiekigry So the idea is fine, but it needs to be done in an optional way, since someone might currently rely on that naming scheme in their existing application / logs.

In the Options Builder, you can call public Builder executor(ExecutorService executor), so maybe add these:

public Builder callbackExecutor(ExecutorService executor) 
public Builder connectExecutor(ExecutorService executor) 

With the defaults matching exactly the existing i.e. newSingleThreadExecutor()

And so starting at line 159 of NatsConnection, that block of code would look like this:

this.executor = options.getExecutor();
this.callbackRunner = options.getCallbackExecutor();
this.connectExecutor = options.getConnectExecutor();

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