Update the PropertyType to make use of FDThreadsafeMutableDictionary

When using the property type in parsing across multiple threads there is an exec bad access ~50% of the time.
Using the thread safe dictionary has addressed this issue for us.
Do you think that we need to change to FDThreadSafeMutableDictionary everywhere or just here for where we create the dictionary? Nothing else really needs to know that it is an instance of FDThreadSafeMutableDictionary.