Previously, SeekableStreamIndexTaskRunner set ingestion state to
COMPLETED when it finished reading data from Kafka. This is incorrect.
After the changes in this patch, the transitions go:
1) The task stays in BUILD_SEGMENTS after it finishes reading from Kafka,
while it is building its final set of segments to publish.
2) The task transitions to SEGMENT_AVAILABILITY_WAIT after publishing,
while waiting for handoff.
3) The task transitions to COMPLETED immediately before exiting, when
truly done.
a83125e4a0