start_time functionality broken

Created by: matthew-d-jones

start_time can optionally be specified in the start message, but doesn't seem to work.

The code in Consumer::addTopic is suspicious, it looks like offsets_for_times() is being called before adding the topic to the PartitionList. Also not sure it is expected to work using RD_KAFKA_PARTITION_UA (unassigned partition) rather than having specific TopicPartitions in the PartitionList. Error handling for the offsets_for_times() call should be added. Should also log an error if the specified start time is in the future.