Async health check

[tomfitzhenry]

For issue #609

Proposed API for users is:

HealthCheckRegistry registry = new HealthCheckRegistry();
ScheduledExecutorService e = new ScheduledThreadPoolExecutor(1);
HealthCheck foo = ...
AsyncHealthCheck asyncFoo = new AsyncHealthCheck(foo, 2, TimeUnit.MINUTES);
e.scheduleWithFixedDelay(asyncFoo, 0, 30, TimeUnit.SECONDS);
registry.register("foo", asyncFoo)

In the above example, the ScheduledExecutorService will execute the health checks with 30s delay. The health check will fail if its cached result is older than 2 minutes.

Design choices:

• Give user control over the async checking, so they can plug it into whatever executor service they have
• Reads must be quick
• Detect if the cached result is too old, and allow user to define what too old is

[tomfitzhenry]

For users that are happy to cede control over the execution of the health check run() methods, perhaps HealthCheckRegister#registerAsync(HealthCheck, int refreshFrequency, TimeUnit refreshFrequencyUnit, int int staleAge, TimeUnit staleAgeUnit) could be added, which managed an internal ScheduledExecutorService

[vbauer]

Any about this PR?... We'd like to have async-feature too

[arteam]

Contributed in #1077