Add a ConcurrentCompositeHealthIndicator

[RikEnde]

See #2652

I submitted a PR for this issue before, but it wasn't very good. I hope this one is better.

Problem:
Right now, invoking the health endpoint lead to a synchronous invocation of all underlying HealthIndicator instance(s) that have been configured. It would be nice to offer an option to run these in parallel instead.

Real world example:
I have a number of HealtIndicator components that poll remote systems and report on latency and status, and the latency really adds up, especially if there are time-outs involved.

Solution:
This PR adds a ConcurrentCompositeHealthIndicator that can run the HealthIndicator instances on the ThreadPoolTaskExecutor.

Usage in code:

    @Bean
    public HealthEndpoint healthEndpoint(HealthAggregator healthAggregator,
                                         HealthIndicatorRegistry registry,
                                         ThreadPoolTaskExecutor executor
                                         ) {
        return new HealthEndpoint(
            new ConcurrentCompositeHealthIndicator(healthAggregator, registry, executor, Duration.ofMillis(1100)));
    }

[snicoll]

@RikEnde did you mean to close this PR?

[RikEnde]

While working on this, I found it could be solved at the application level once we move to spring boot 2.1.x

@snicoll I'm really sorry for wasting your time. I felt less good about this PR every day, and now that I have a workaround, I can't justify working on it during the day.

[CLOUGH]

@RikEnde could you share the workaround that you found.

[RikEnde]

@RikEnde could you share the workaround that you found.

The code from this PR could be implemented entirely in domain code, requiring no framework changes. However, this was 2 years old. It doesn't even compile against the current version.