Support indexer_stats log file in ns_server

As of couchbase server 6.5, indexer stats are logged to indexer.log file in a single json blob. In Cheshire-Cat, we are improving this by
1. Logging stats to a different file (indexer_stats)
2. Exploit stats deduplication over time.

This will be achieved with the help of https://github.com/couchbase/logstats

The new file proposed here will be managed by indexing service. But for that indexer will need help from ns_server for the following.
1. Provide the path to ns_server log directory as command line argument
2. Improve cbcollect to get indexer_stats