Change sensor data transfer

 - Keep binary file always open (mode "a+b"); don't need to reopen each time it is accessed
 - Fill query buffer with most recent datapoints (as opposed to trying to transfer datapoints in sequential order)
   - Better because it keeps client as up to date as possible
   - Rather than client getting increasingly outdated data

TODO:
 - Test to see if buffers in front of writing to file worth it
   - Probably will be; saves from having to unlock mutex on every single data point
 - Merge Query and FCGI stuff