Changed the default converters to use local instead of class variables

[mircopz]

Fix as indicated in #1103

The value returned by a converter must not be a reference to a static variable. Instead, in this project the default converters use a class variable (self.dict_result) to process the received message, and return it at the end.

So if 2 different messages are queued in __converted_data_queue while __send_to_storage thread is in its 0.2s sleep, when it wakes up it found a queue of size 2 but both elements have the same reference, whose value is both times the last value assigned to that variable (the last converted message).

The solution is to use a local variable inside the convert function (dict_result instead of self.dict_result).