Gracefully Handle DPM Switches While Processing Data Logger Requests

[beauremus]

The client needs to track data logger responses so that in the case of a DPM switch it can change the requests to the next reasonable value.

For example, if you make a request for data for the past month and you have collected data for only 15 days. When the DPM switch happens the client would submit a new request to the new DPM for the past 15 days.

[rneswold]

@beauremus, @charlieking65 and I had a discussion about this and came up with the following strategy (in case someone else besides us wants to tackle it before we do):

• The dpm object keeps all the DRF requests -- even after sending them to DPM -- so that it can resubmit them to another DPM if the one it's connect with goes away.
• As replies arrive for data logger requests, the dpm module will modify the DRF request -- reducing the time range to exclude data that has already arrived.
• If the module has to switch DPMs, the remaining time range will be resubmitted to the new DPM.