Add the compliant "Direct" doc-value format

In an effort to keep the "Direct" doc-value format in future release, we moved them to doc-value iterators. Any review would be appreciated.

Main changes include:

• Use DocValuesIterator instead of LegacyDocValues.java in DirectDocValuesProducer;
• Embed iterator implementation inside DirectDocValuesConsumer for code reuse;
• All test cases have been run successfully and included in pull request;

P.S. Our business scenario:

• Save the inventory in Direct doc-value format;
• Update them in memory with high-frequency without persistence;
• Load inventory from data storage during app ignite;