Roxie dali notification issues

Some issues with how Roxie handle notifications of package changes:

1. Typically notifications come in for PackageMap and then PackageSet in rapid succession, resulting in multiple threads being kicked off. This increases the load on dali (and MAY result in indeterminate end-state, depending on which completes first.

2. Roxie is creating new subscriptions to PackageSets in response to the notifications - these may themselves receive notifications resulting in still more threads.