Add CompositeHealthIndicator

Add CompositeHealthIndicator that allows multiple HealthIndicators
to be combined into a single result.

Fixes gh-782
Add multi-datasource health indicator support

Update EndpointAutoConfiguration to support multiple datasources.

Fixes gh-783
Merge pull request #780 from tjf/1.0.x

* pull780:
  Add multi-datasource health indicator support
  Add CompositeHealthIndicator