session_data__unpack() function

Merge earlier message into a latter message. For numeric types and strings, if the same value appears multiple times, the parser accepts the last value it sees. For embedded message fields, the parser merges multiple instances of the same field. That is, all singular scalar fields in the latter instance replace those in the former, singular embedded messages are merged, and repeated fields are concatenated. The earlier message should be freed after calling this function, as some of its fields may have been reused and changed to their default values during the merge.