Thread safety in DefaultPropertyValueConverterFactory

Description

Problem:
Intermittent failure upon startup in solutions that use PropertyList. The error is a "IndexOutOfRangeException: Index was outside the bounds of the array" from the DefaultPropertyValueConverterFactory.

Cause:
While difficult to consistently reproduce the error, it is probably caused by the use of a standard dictionary inside a singleton.

Solution:
Changed to a Concurrent Dictionary.