Workers don't get new IP package from frontier

[MichaelRoeder]

Problem

During the 3rd run, the workers were throwing exceptions because the frontier stopped answering, e.g.,

{"log":"2017-10-02 13:07:46,583 [main] ERROR [o.h.c.r.RabbitRpcClient       ] - \u003cException while sending query. Returning null.\u003e\n","stream":"stdout","time":"2017-10-02T13:07:46.585655178Z"}
{"log":"java.util.concurrent.TimeoutException: Timeout waiting for task.\n","stream":"stdout","time":"2017-10-02T13:07:46.585709808Z"}
{"log":"\u0009at com.google.common.util.concurrent.AbstractFuture$Sync.get(AbstractFuture.java:269)\n","stream":"stdout","time":"2017-10-02T13:07:46.585717681Z"}
{"log":"\u0009at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:96)\n","stream":"stdout","time":"2017-10-02T13:07:46.585725113Z"}
{"log":"\u0009at org.hobbit.core.rabbit.RabbitRpcClient.request(RabbitRpcClient.java:156)\n","stream":"stdout","time":"2017-10-02T13:07:46.585732453Z"}
{"log":"\u0009at org.aksw.simba.squirrel.components.WorkerComponent.getNextUris(WorkerComponent.java:83)\n","stream":"stdout","time":"2017-10-02T13:07:46.585739529Z"}
{"log":"\u0009at org.aksw.simba.squirrel.worker.impl.WorkerImpl.run(WorkerImpl.java:80)\n","stream":"stdout","time":"2017-10-02T13:07:46.585746485Z"}
{"log":"\u0009at org.aksw.simba.squirrel.components.WorkerComponent.run(WorkerComponent.java:71)\n","stream":"stdout","time":"2017-10-02T13:07:46.585753514Z"}
{"log":"\u0009at org.hobbit.core.run.ComponentStarter.main(ComponentStarter.java:67)\n","stream":"stdout","time":"2017-10-02T13:07:46.585760435Z"}
{"log":"2017-10-02 13:17:48,586 [main] ERROR [o.h.c.r.RabbitRpcClient       ] - \u003cException while sending query. Returning null.\u003e\n","stream":"stdout","time":"2017-10-02T13:17:48.587168887Z"}
{"log":"java.util.concurrent.TimeoutException: Timeout waiting for task.\n","stream":"stdout","time":"2017-10-02T13:17:48.587222859Z"}
{"log":"\u0009at com.google.common.util.concurrent.AbstractFuture$Sync.get(AbstractFuture.java:269)\n","stream":"stdout","time":"2017-10-02T13:17:48.587230644Z"}
{"log":"\u0009at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:96)\n","stream":"stdout","time":"2017-10-02T13:17:48.587238083Z"}
{"log":"\u0009at org.hobbit.core.rabbit.RabbitRpcClient.request(RabbitRpcClient.java:156)\n","stream":"stdout","time":"2017-10-02T13:17:48.587245243Z"}
{"log":"\u0009at org.aksw.simba.squirrel.components.WorkerComponent.getNextUris(WorkerComponent.java:83)\n","stream":"stdout","time":"2017-10-02T13:17:48.587252323Z"}
{"log":"\u0009at org.aksw.simba.squirrel.worker.impl.WorkerImpl.run(WorkerImpl.java:80)\n","stream":"stdout","time":"2017-10-02T13:17:48.587259355Z"}
{"log":"\u0009at org.aksw.simba.squirrel.components.WorkerComponent.run(WorkerComponent.java:71)\n","stream":"stdout","time":"2017-10-02T13:17:48.587266352Z"}
{"log":"\u0009at org.hobbit.core.run.ComponentStarter.main(ComponentStarter.java:67)\n","stream":"stdout","time":"2017-10-02T13:17:48.587272977Z"}

This could be caused by an empty queue in the frontier. But this should be answered in a different way. It could be helpful if the frontier regularly logs its status (queue length, number of IPs blocked, etc.).