JobMaster cannot be restarted

Since we are reusing the DefaultLeaderRetrievalService for finding the resource manager in the JobMaster and since the leader retrieval service cannot be reused, it is not possible to restart the JobMaster. This causes Flink to fail in case that the JobMaster regains the leadership in Flink versions <= 1.12. The problem no longer occurs for newer versions because of FLINK-11719.