avg function: small refactoring to improve code reuse and fix possible
ArithmeticException on non distributed executions
Merge pull request #2625 from fabriziofortino/develop

avg function: small refactoring to improve code reuse and fix possible
removed code not used anymore from binary serialization.
implemented first cached string support in the binary serialization.
fixed string cache persistence, add string cache persistence test
fixed wrong optimized serialization of the schema