cbdatasource.Close() might not be synchronous (according to code comments)

According to this code:

// TODO: By this point, worker goroutines may still be going, but
// should end soon. Instead Close() should be 100% synchronous.
return nil

While running Sync Gateway integration tests in a loop where:

• Flush bucket
• Do sync gateway operations
• Verify operations are successful
• Repeat 

We're seeing "DCP Rollback" warnings in the logs, and one theory is that there is residue from a previous test that is getting called back, possibly due to cbdatasource.Close() being asynchronous.