box_monitor: history data from sensors while offline

[neaxi]

When the board can't reach the network (unavailable SSID) should we cache the sensor data and upload them all once the connectivity restores?
Possible approach

• implement buffer
  • limit size / how much historical values will be stored
    • there should be some sort of a limit to prevent memory exhaustion, otherwise we won't have enough memory to initiate HTTPS connection (see micropython/micropython-esp32#209 (comment))
  • must record timestamp when the entry was recorded
    • how to tell the API the measurement was taken at a different time than time of upload?
• once the connectivity restores, start processing buffered data
• while the buffered data are being processed, keep adding current measurements to the buffer
  • example:
  • data are measured once per 60 sec
  • API throttling is set to 15 sec
  • we can't upload everything prior next measurement interval
• parallel threads?
  • one handling data upload and one that keeps measuring current events