[FLINK-5863][queryable state] Add the serialization of list states in KvStateRequestSerializer

[shixiaogang]

1. Add serializeList() in KvStateRequestSerialization
2. Modify the unit tests of KvStateRequestSerialization, without the access to protected methods.
3. Move KvStateRequestSerializationRocksDBTest from package org.apache.flink.test.query to org.apache.flink.contrib.streaming.state.

[uce]

If I understand this correctly, this gets rid of duplicated code. I think this is in general very valuable, but the problem is queryable state right now is that we need to refactor this for 1.3 anyways. Currently, too many flink-runtime components are exposed to the user. I'm working on a FLIP to improve the user APIs for 1.3 and deprecated (or maybe even remove) the current APIs. This will therefore become obsolete. What do you think? Should we close this PR and issue and you help on the proposed changes instead?

[shixiaogang]

The PR is just some cleaning of the code. Now that we are planning to refactor the implementation, I agree to close the PR and I am very willing to contribute to the FLIP.