Processor can be scheduled to start running while it is still in a 'STOPPING' state

After the refactoring of the Processor Lifecycle that happened in 0.6.0, a Processor can be told to start while it is still 'stopping'.

StandardProcessorNode.verifyCanStart() has the following logic:

        if (this.getScheduledState() == ScheduledState.RUNNING) {
            throw new IllegalStateException(this + " cannot be started because it is already running");
        }

However, if the state is 'stopping', it also should not be able to start. This wasn't accounted for originally because there was no 'stopping' state. There was only RUNNING and STOPPED; if not RUNNING, it was assumed to be stopped. However, now that there is a STOPPING state, we should instead verify:

        if (this.getScheduledState() != ScheduledState.STOPPED) {
            throw new IllegalStateException(this + " cannot be started because it is not completely stopped");
        }