CBG-3857 Mark database state of starting more clearly

[torcolvin]

This PR only changes the messages of a 503 error.

Put database in starting state, so while it is starting up between getting added to sc.databases_ and StartOnlineProcesses requests will return DB is %v - try again later instead of DB is currently under maintenance. Make sure that in the case of start offline, it will not transition through Starting state.

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/2365/