Plan to support cellular data caps

[tve]

The main cases where data caps are expected to be necessary:

• When we are getting excessive noise from one or more antennas on Lotek freqs which result in a large number of pulses recorded daily. Or there are just a large number of tags deployed nearby which being detected each data (e.g.; colonial species).
• There is some bug causing a lot of messages to be logged repeatedly.

Ideas:

• user enters a data cap per month, e.g. 500MB and a start day-of-the-month
• the SG calculates an average daily cap based on data remaining in the month
• when the SG reaches the data cap it stops all outgoing communication, you can still SSH into the SG or connect to its web interface
  the next day the SG resumes "normal operation"
• in terms of uploading data files: uploads happen hourly, each time they pick a day at random and then upload all files for that day
• all the data cap stuff would be for one interface only, e.g. typically the cellular interface

Open issues:

• Is it necessary to prioritize data upload over log file uploads in some way? Hopefully not, the log file uploads are throttled so even if they are huge there is quite some opportunity for data uploads to happen, but maybe each (hourly) log file upload needs to be limited to, say, 1/10th of the data cap...
• What to do when a data upload for a specific date will exceed the data cap? Easiest is to skip it and presumably another day will be picked the next hour and it may fit into the remaining data allowance...
  This has the effect that a day that produces more data than the data cap will never be uploaded, i.e., will have to be retrieved manually (or the data cap increased), maybe that's OK as long as it's visible/alertable?