KAFKA-20304: Implement all required ReadOnly*Facade classes for headers stores

[aliehsaeedii]

This PR

• adds all ReadOnly*Facade classes required to wrap headers store for
  backward compatibility
• refactors ReadOnly*Facade classes to use generic infrastructure with
  converter functions

Reviewers: Matthias J. Sax matthias@confluent.io

[aliehsaeedii]

I'm wondering if it makes sense to remove

• ReadOnlyKeyValueStoreFacadeTest (exists)
• ReadOnlyWindowStoreFacadeTest (exists)
• KeyValueIteratorFacadeTest (exists)

just to be consistent.

[mjsax]

Yes, I had the same thought about removing these tests even before I read yoru comment. We can do a follow up PR or add here. Whatever you prefer.

As a matter of fact, I am even wondering if we need to add all these sub-classes of the GenericXxx classes? How often do we use these sub-classes As you test code shows, we can easily instantiate GenericXxx directly, so I am wondering if we gain much by having all the sub-classes or if we could drop them, and let each user just use GenericXxx directly?

[aliehsaeedii]

Yes, I had the same thought about removing tests even before I read you comment. We can do a follow up PR or add here. Whatever you prefer.

As a matter of fact, I am even wondering if we need to add all these sub-classes of the GenericXxx classes? How often do we use these sub-classes As you test code shows, we can easily instantiate GenericXxx directly, so I am wondering if we gain much by having all the sub-classes or if we could drop them, and let each user just use GenericXxx directly?

The only real benefit of the subclasses is for TopologyTestDriver inheritance:

• TopologyTestDriver.KeyValueStoreFacade extends ReadOnlyKeyValueStoreFacade
• It needs the protected TimestampedKeyValueStore<K, V> inner field to call write methods
  But you are right, we could refactor this pattern to use composition instead of inheritance

[mjsax]

I don't think we need to have this as a member, but KeyValueStoreFacade can just inherit from GenericReadOnlyKeyValueStoreFacade directly.

I am going to just push a change for this (also removing unnecessary tests -- don't need to re-test what we inherit from GenericReadOnlyKeyValueStoreFacade in KeyValueStoreFacadeTest.