Extend exception handling in ConcurrentUtils to runtime exceptions

ConcurrentUtils already has methods for handling ExecutionException}}s. So far such exceptions are transformed into {{ConcurrentException which is a checked exception.

In modern frameworks there is a tendency to avoid checked exceptions and use runtime exceptions instead (e.g. Spring or Hibernate). Thus it makes sense to provide the transformation to unchecked exceptions, too, to support client code that does not want to deal with checked exceptions.

This is a proposal to introduce a new runtime exception class ConcurrentRuntimeException and to duplicate the functionality in ConcurrentUtils related to exception handling to also support the new runtime exception class.