Adding nodes to a 5.0.0 cluster can return "None" as error, never recovering

Without SSL, Adding 2 nodes in Spock GA then KV Async operation spews "None" as error and throughput drops slightly as apposed to expected high throughput after adding 2 nodes.

Steps to reproduce:

1. Spock cluster with 2 nodes (1 kv, 1 kv/index/query/fts), with 2 buckets. (test was against the first bucket)
2. keep KV operation against the Spock cluster and add 2 nodes and rebalance.

Expected : No error and throughput increase.

Actual: "None" error message with throughput slightly decrease.

http://sdk-testresults.couchbase.com.s3.amazonaws.com/SDK-SDK/CB-5.0.0-xxxx/Rb2In-HYBRID/10-03-17/071054/99763117665d02947674a88d7c6ac0f3-MC.html