New oracle needed to detect if the changed config is rolled out

[tylergu]

The bug in zookeeper #59 is caused because the config is translated into a configMap object, but the application is not restarted to reflect the changed configs. koperator also had a very similar bug like this.

In our current oracles, we are not able to detect such bugs. In the input delta, Acto would see that a config field is changed. Then Acto tries to find a matching delta in the application state. It successfully finds a matching delta in the configMap object, thus passes the check.

To detect such bugs, I can think of two ways:

1. We need to first recognize which field represents the application config. Then when this field changes, we apply an additional oracle to check if application restarted or not. This is heuristic, because not all applications/configurations need to restart the application to roll out the changed config.
2. The second way is to provide an interface to allow users to do special checking for certain fields. Users could register a callback function to the config field as an extension of the oracle. The callback function will be called as part of the oracle when the test case change the config field. Inside the callback function, it implements the mechanism to check whether the changed config is applied on the application.

[tylergu]

Added custom oracle