Spawn tasks using Worker API

[eyalroth]

Fixes #75.

This makes the plugin tasks (ScoverageReport and ScoverageAggregate) run using Gradle's worker API, which provides an ability to run them with a custom classpath without affecting the main Gradle classpath/classloader.

Logging may change slightly, as the the worker API does not expose a way to use the project's logger (see gradle/gradle#2678).

Another aspect that may change is the performance of the Gradle build. #75 initially addressed the performance hit caused by spawning each task in its own process, so the fix then (version 3 I think) was to run in the main Gradle process, which introduced the pollution of the classpath/classloader.

Looking at the worker API documentation about isolation modes, it's implied that running with a separate class loader will not spawn another process (otherwise why would there be another isolation mode just for running in a separate process?), so my hopes are that the performance is not impacted in a significant way.

[eyalroth]

Looks like the Worker API is causing metaspace memory issues, see gradle/gradle#15200, gradle/gradle#8291, gradle/gradle#8221.

I'm placing the PR back into draft mode until Gradle will provide a better way to run tasks with an isolated classpath.