INT-2889 Fix Concurrency Problem in Type Converter

In the BeanFactoryTypeConverter, when falling
back to a PropertyEditor (when the source is non-
String and the target is String), the setValue() and getAsText()
methods are used.

This is not thread-safe and one thread might get another's
converted value.

Synchronize the use of the PropertyEditor.

Add test, that reliably reproduces the problem, to verify the
fix.
Merge pull request #710 from garyrussell/INT-2889

* INT-2889: Fix Concurrency Problem in Type Converter