SAMZA-2516: Migrate BaseKeyValueStorageEngineFactory to be an abstract class instead of trait

[cameronlee314]

Issues:

1. If using Scala 2.11, there is a compile error when trying to use a Java class extend a Scala trait with implemented methods. BaseKeyValueStorageEngineFactory can be considered part of the Samza API, but it is a Scala trait with an implemented method, so that restricts extension.
2. It would be good to migrate away from Scala code in general, for consistency with other new Samza code.

Changes:

1. BaseKeyValueStorageEngineFactory is now an abstract class written in Java.
2. Added some unit tests for BaseKeyValueStorageEngineFactory.
3. The functionality is intended to stay the same.

Tests:

1. Added unit tests
2. Some tests in samza-test use in-memory key-value storage, and those tests pass.
3. Deployed job with "drop.large.messages" enabled and verified that messages get dropped
4. Deployed job with "disallow.large.messages" enabled and verified that exception is thrown
5. Deployed job with changelog enabled and verified that changelog was being written to

API changes:
BaseKeyValueStorageEngineFactory is no longer a Scala trait, so that could potentially impact existing inheritors:

1. Any existing Scala classes which uses the with syntax to mix in BaseKeyValueStorageEngineFactory will need to be updated to use BaseKeyValueStorageEngineFactory as an abstract class instead. If an existing Scala class used extends BaseKeyValueStorageEngineFactory (such as how InMemoryKeyValueStorageEngineFactory and RocksDbKeyValueStorageEngineFactory uses BaseKeyValueStorageEngineFactory), then it should not need to change.
2. Scala 2.12 compiles traits into interfaces with default methods, so a Java class built against the Scala 2.12 version of BaseKeyValueStorageEngineFactory won't be able to use it as an interface. A Java class will need to use BaseKeyValueStorageEngineFactory as an abstract class. I suppose BaseKeyValueStorageEngineFactory could work as an interface with a default implementation for getStorageEngine, but it seems like abstract class fits the pattern better, since getKVStore shouldn't be a public method.

Upgrade instructions:
Any custom implementation of BaseKeyValueStorageEngineFactory needs to treat it as an abstract class, not a trait/interface. If it is necessary to make changes, only class inheritance structure should need to be changed. The functionality/signature of getStorageEngine and the usage of the getKVStore abstract method are the same, so those pieces should stay the same.

[PanTheMan]

Looks good overall. I just have some minor comments

[PawasChhokra]

Just left a few minor comments. Otherwise looks good to me.

[rmatharu-zz]

took one pass

[PanTheMan]

lgtm

[rmatharu-zz]

lgtm