change the dict in which parsed properties are stored from homogeneous

change the dict in which parsed properties are stored from homogeneous
to heterogeneous. A leak occured in
teshsuite/msg/storage/storage_basic.c after the addition of a call
MSG_storage_set_property_value that adds a new property to the dict. The
new element doesn't appear to be freed when freeing the resources. After
several tries, the proposed modification solves the issue without
breaking something else.