Specify warmup instead of cold starting the ExecutorServiceHandler

[Tzal3x]

So far workers are created on demand from the eshandler, meaning that there is a cold start. With the current implementation, it will take a few requests for the eshandler to adapt to the load and have available workers waiting to accept transactions.

In most cases it would be useful to "warmup" the eshandler to be ready to execute transactions from the moment of initialization, without the need to accept requests first.

i.e. we could define a warmup function that adds workers to the _workersQueue a-priori, during initialization.