INT-2922 Fix TypeConverter Concurrency Issue

During initialization, it is possible for type conversion to fail because a second thread accesses a partially built list of PropertyEditors. This is because getDefaultEditor(Class) is not thread-safe.

* Add a test with mocks and spies to verify concurrent access to the method occurs before the fix, and not after the fix.
* Synchronize the call to getDefaultEditor(), when called from canConvert() and convertValue().
* Only synchronize the call until we have invoked the method at least once.
Merge pull request #739 from garyrussell/INT-2922

* garyrussell-INT-2922:
  INT-2922 Fix TypeConverter Concurrency Issue