Guaranteed sensor readings transfer

[neilh10]

For any sensor readings, it is stored to a local uSD, and then should guarantee the orderly transfer of data to the cloud database if there is a comms channel.

It would be valuable in transferring current management data on every connection for user attempting to monitor status of nodes. This could be LiIon V status, outstanding readings.

Background: ModularSensors makes a best effort to transfer sensor readings to a destination.
If a fail indications is received, for whatever reason, there is no retry mechanism.
When a sensor readings is complete, it writes it to a local uSD card, and then attempts a connection.
For any user visiting a geographical site, they should be able to retrieve the SD card.
This presents problems for managing small unlabeled physical uSD cards.
If the readings need to be available online, it would be possible for some users to merge the uSD with datasets online, but it could be complex, and time consuming.

EnviroDIY#194

[neilh10]

This has been extensively tested, and with low battery checking https://github.com/neilh10/ModularSensors/releases/tag/v0.27.5.release1_210203