Allow ExecutorService to be injected

[boaty82]

Please, please, please can this be injected through an overloaded constructor, and make the existing constructors chain on with a default value as per current defaulting logic.

I have need to use this class with another executorservice/threadfactory and it's looks like I'm going to have to now duplicate a lot of code

spring-integration/spring-integration-core/src/main/java/org/springframework/integration/support/leader/LockRegistryLeaderInitiator.java

Lines 78 to 79 in e53e894

	private final ExecutorService executorService =
	Executors.newSingleThreadExecutor(new CustomizableThreadFactory("lock-leadership-"));

[artembilan]

Yes, we can do that.

CONTRIBUTING is welcome!

[boaty82]

Okie dokie I shall certainly do so!

[boaty82]

I did start refactoring the class too, to make it smaller and have a single responsibility, but felt I should possibly do this under another pull request.

May have some other feature requests (with PoCs) and some other helpful/reusable implementations I can submit

[artembilan]

I don't find that class too bad by design so we need to refactor it, but you are feel free to share your vision anyway. However right: let's do that in the separate PR!

[boaty82]

I just felt the class in particular didn’t follow the open/closed principle. It’s complexity is quite large too. But yes agree a different PR