[3.1.4 fix] CBG-3777 make sure cbgt always sets kv_pool_size=1

[torcolvin]

This forces kv_pool_size to 1 as required by DCP to have unique connection names. This is a much simplified version of #6680 that should be low risk for inclusion in a maintenance release.

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/2295/