Since the event loop thread is owned by the DefaultSocketProvider, exceptions are not being propagated to the client.run() thread as before and the Sync_MultipleSyncAgentsNotAllowed test needed to be commented out. Add an error handler to the default sync socket that can be used during the test to receive errors that occur during the test so this test can be updated to use this error handler.

The BindingCallbackThreadObserver is used by the default socket provider implementation to provide the following notifications back to the SDKs:

• Event loop thread started
• Event loop thread exiting
• Error occurred on event loop

Currently, the BindingCallbackThreadObserver is a global object that requires all of the functions to be implemented. Update this class to be optionally provided to the SyncClientConfig as a shared object that can either be subclassed or instantiated using a set of (optional) function pointers.

This task was originally going to be addressed in PR #6156, but these changes will be moved to another PR.