Fix the BufferGroup wait problem.
The problem can be observed with the ConcurrentRegisteredBufferTest. Buffer notifications are not global and a notified Fiber may not wakeup on the worker where the notification happend.
Waiting for a RegisteredBuffer is not supported anymore. The user can try to get one of the current worker or call
RegisteredBuffer::getOrAlloc() which will use a free registered one or tranparently allocate a new buffer.