Flush changes at the end of Modificator

[aslakknutsen]

What can be improved

In the situation where a Ref is updated in a session the Locators will create two actions for most Resources, DELETE/REVERT and CREATE/MODIFY. This will trigger in the current setup a sequence of events:

read - revert - save - read - modify - save

In a lot of the cases like VirtualService, Gateway and DestinationRule this will recreate the exact same output. That means we could detect that both 'create' and 'delete' will happen to the same resource and likely to get away with a sequence of events like:

read - (revert - modify) - save?

Why

This will lower the amount of calls required to the API server and config flushes needed by istiod.

There is a slight chance that this will also fix a strange race like condition observed on the Brno cluster.