Selection of non-disk RRD strategies

There currently exist two RRD strategies which do not write to disk, and only write to the TCP stream: TcpRrdStrategy and QueuingTcpRrdStrategy. However, neither of these can be chosen by the existing RRD properties, "useTcp" and "useQueue". We need to add a third option "useRrd" (or "useDisk"?") to these. useRrd=true will select the existing four strategies with the current useTcp/useQueue options. A combination of (useRrd=false, useTcp=true, useQueue=false) will choose the TcpRrdStrategy and (useRrd=false, useTcp=true, useQueue=true) will choose the QueuingTcpRrdStrategy. If useRrd and useTcp are both false, no strategy will be selected, since nothing is being done there (or should that be NullRrdStrategy).