Race condition in request id generation of GrpcStateRequestHandler

There is a race condition in GrpcStateRequestHandler which surfaced after the recent changes to process append/clear state request asynchronously. The race condition can occur if multiple Runner workers process a transform with state requests with the same SDK Harness. For example, this setup occurs with Flink when a TaskManager has multiple task slots and two or more of those slots process the same stateful stage against an SDK Harness.

CC robertwb