Refactored/extended parallelism API

Changes:
* use `java.util.concurrent` API rather than manual thread creation
* allow to choose between static and dynamic query->worker distribution
* instantiation via builders (`ParallelOracleBuilders`)
* `ParallelOracle` is now named `StaticParallelOracle`, `ParallelOracle` is superinterface for parallel oracle (exposes `shutdown` and `shutdownNow` methods)