Swap FTS node rebalance failed (boltdb panic)

Build 6.0.0-1693

Observed that swap rebalance of FTS node in high bucket density test failed with following error-

Rebalance exited with reason {service_rebalance_failed,fts, {rebalance_failed, {service_error, <<"nodes: sample, res: (*http.Response)(nil), urlUUID: monitor.UrlUUID{Url:\"http://172.23.96.20:8094\", UUID:\"050b9271854fba4a40721562e160e9be\"}, kind: /api/stats, err: Get http://%40fts-cbauth:51f01258286dc0e326539b393b07c7c1@172.23.96.20:8094/api/stats: EOF">>}}}

The setup has 30 buckets and 1 FTS index per bucket and no its queries.

Tried swap rebalance again on same setup got same error.

Logs-
FTS node which is rebalanced out- https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-arke-multi-bucket-250/172.23.96.20.zip
FTS node which is rebalanced in- https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-arke-multi-bucket-250/172.23.97.15.zip
Analytics node- https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-arke-multi-bucket-250/172.23.96.23.zip
Eventing node- https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-arke-multi-bucket-250/172.23.97.177.zip
Index+query node- https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-arke-multi-bucket-250/172.23.97.19.zip
Index+query node- https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-arke-multi-bucket-250/172.23.97.20.zip
KV node- https://s3.amazonaws.com/bugdb/jira/30_bucket_reb_logs/collectinfo-2019-01-09T041824-ns_1%40172.23.97.12.zip
KV node- https://s3.amazonaws.com/bugdb/jira/30_bucket_reb_logs/collectinfo-2019-01-09T041824-ns_1%40172.23.97.13.zip
KV node- https://s3.amazonaws.com/bugdb/jira/30_bucket_reb_logs/collectinfo-2019-01-09T041824-ns_1%40172.23.97.14.zip