CBG-788 set agent priority to medium for caching feed/CE import feed

[torcolvin]

Pre-review checklist

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

Integration Tests

• [x] xattrs=true https://jenkins.sgwdev.com/job/SyncGateway-Integration/CBG-788/