Fix dispose unwinding for Consumer
Fix dispose unwinding for Consumer

Wait on the polling tasks to exit before disposing the cancel token.

BDeus/KafkaNetClient,Jroland/kafka-net,nightkid1027/kafka-net,EranOfer/KafkaNetClient,CenturyLinkCloud/kafka-net,geffzhang/kafka-net,PKRoma/kafka-net,gigya/KafkaNetClient,martijnhoekstra/kafka-net,aNutForAJarOfTuna/kafka-net,bridgewell/kafka-net