more cleanup on jacoco plugin

- configure decent defaults for JacocoReport and JacocoMerge
- add incubating annotation to all public classes
- move implementation classes into internal
- use Gradle logger instead of groovy SLF4F annotation
- add jacoco task package to default imports
Add jacoco chapter to userguide and add DSL documentation of jacoco classes

TODO: add jacoco to release notes
minor fixes on Sonar docs.