Chunk server: rework client state machine ops handling. Only keep "depending" ops, i.e. write prepare with no reply (these are only used by the fairly old qfs clients, and exist only for backward compatibility) in the ops queue, and just keep "in flight' count for all other ops. This optimization should help to avoid any linear searches in the op queue with recent enough clients, thus reducing cpu utilization. Write append with chunk server connection pooling is the primary optimization target, as out of order append completion is expected under normal circumstances. The linear queue search could explain higher than expected chunk server cpu utilization with write append, especially in the case with chunk server connection pooling.