Allow requesting online data from a nds2 server at sub second strides.

There is work to have a nds2 server consume the CDS daq data stream at 16hz. With minor changes the client can be made to request data to be delivered at 16Hz.

Allow -1 to be used to signal the nds2 server for fast data delivery.

We will also need to catch errors when requesting a -1 stride and reissue the request at a stride of 1 to deal with older servers.