Improve Thrift map deserialization code

Improve Thrift map deserialization code

Summary: Instead of copy-constructing map values into the map, alter the code such that we insert default-constructed elements into the map and then deserialize them by reference.

Reviewed By: hzhao

Test Plan: Ran the test in test/cpp which include serialization and deserialization of nested maps.

git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665448 13f79535-47bb-0310-9956-ffa450edef68