Connection leak in index scanner

How to reproduce:
1. Set indexer log level to debug (This is very hard to reproduce with log level=info)
2. Run cbindexperf with concurrency=200 couple of times with total requests=100M
3. While cbindexperf is running, kill the process by Ctrl-C
This causes many requests to issue shutdown in the scan pipeline.
4. grep num_connections stats to make sure that it becomes zero after all cbindexperf processes are exited.