Allow Client to Provide ExecutorService

[kkoz]

Picks up #10
Related to glencoesoftware/omero-ms-image-region#50

Previously, when using the HSBRenderingStrategy, the class would create its own ExecutorService here:

omero-renderer/src/main/java/omeis/providers/re/HSBStrategy.java

Line 332 in 5c92fe8

ExecutorService processor = Executors.newCachedThreadPool();

newCachedThreadPool returns a ThreadPoolExecutor which will create an unbounded number of threads. The combination of having one ThreadPoolExecutor per Renderer plus that pool always being unlimited in size means that clients can unintentionally create huge number of threads.

This PR changes that by providing the client with the option to create and manage the ExecutorService.
With this PR, Renderer takes an ExecutorService as an argument and passes it through to the HSBRenderingStrategy for use in rendering. This means the calling code gets to determine the quantity and properties of the thread pools, and can therefore put hard limits on how many threads might get created.

https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/util/concurrent/ThreadPoolExecutor.html has good information about how to create and manage ThreadPoolExecutors to achieve the desired behavior.

[sbesson]

Closing as per glencoesoftware/omero-ms-image-region#50 (comment)