Termination of long-running query executions

[hartig]

It would be useful for the engine to be able to terminate long-running query executions after a configurable amount of time. To achieve this we need two things:

1. some approach to terminate an ongoing query execution in an orderly fashion (i.e., all the threads that where spawned by the execution and all elated CompletableFutures should be interrupted and terminated).
2. a monitoring component that checks for long-running query executions and invokes the termination functionality for the ones that have reached the timeout limit.

[hartig]

A basis for the monitoring component may be the data structure considered in #95