Add auto-configuration for general use TaskExecutor

[vpavic]

This PR adds auto-configuration for general use TaskExecutor as described in #5082.

Since Boot 1.4.0 uses Spring 4.3.0, the auto-configured TaskExecutor instance will also be picked up and used in @EnableAsync scenarios.

I'll update the documentation after the review.

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[snicoll]

I could see a spring.task.pool.max-size in there to avoid multiple related properties in the root namespace.

[vpavic]

Agreed.

[vpavic]

I've updated the PR with pool configuration properties holder, as per @snicoll's suggestion in comments.

This also resolves #1563.

[philwebb]

Thanks. We're still working though some design considerations for how we'd like the TaskExecutor to work and I'm afraid It's likely that we'll want to fix this a different way.

[vpavic]

@philwebb Thanks for the feedback. Could you share some details on the approach you'd like to take?

I've got two other PRs (#4299 and #5066) that are somewhat dependent on the availability of general TaskExecutor and would like to know what are the chances they get included in the final 1.4.0 milestone?

[philwebb]

@vpavic we've not dug into the details yet I'm afraid. All we know is that we need to tread carefully and that we will need to liaise with some of the other teams (especially spring cloud).

I know that's not very helpful, but we've been burnt in the past by adding features that had knock on effects later. It may take a little time to sort this one out.

[vpavic]

@philwebb Thanks for clarifying.