Add API to prestart threads in threadpools

For my use case I would like to ensure that when I create a thread pool with min_threads > 0, that the minimum number of workers are created immediately.

The Java interface for ThreadPoolExecutor calls this "prestart". For example: prestartCoreThread and prestartAllCoreThreads: https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ThreadPoolExecutor.html#prestartCoreThread--

I have a draft PR (here) that implements a similar API for the CRuby implementation (I left the JRuby implementation for later). It adds both methods, as well as a prestart option to the initializer.

Is this an API change you would consider accepting?

Start by reviewing the draft PR linked in the issue, especially its CRuby implementation, and compare the proposed API with the existing JRuby thread-pool implementation. No files or tests are named in the issue. Done means the prestart API and initializer option have an agreed design and supported implementations for the relevant runtimes.