Avoid double converter registration warning.
Please remove this annoying message during converter registration:
WARNING: Converter with signature ConverterSignature [schema=.rct.FrameTransform, datatype=class rct.Transform] already registered in DefaultConverterRepository. Existing entry will be overwritten!
Because there is no method to check if a converter was already registered by other components within the same vm there is no way to avoid this message without centralized converter loading.
Make DefaultConverterRepository thread-safe
Use a ConcurrentHashMap instead of a normal one. Also reduce the
visibility of the warning message in case a converter is replaced and
document the replacing behavior.
(cherry picked from commit 0dce4827849d40bc3fcfbf6b3427b271862e2d67)
DefaultConverterRepository: expose registered converters
Adds a method to inspect the registered converters.