Decentralize reader's database

[Sashkoiv]

Currently, all requests from the reader go to the central node which is Raspberry Pi. Then the Raspberry Pi does the magic and replies with whether to unlock the equipment (switch the relay). In this scenario good connection as well as central node stability is vital.

The suggestion is to store a database of allowed users on the reader directly. This should provide more stability and reliability and at the same time untie the reader from infrastructure.

In order to maintain the local reader's database up to date I'd suggest together with the heartbeat packet sending the status of the main database (hash-sum, last update timestamp, request of update, TBD).

By the database, I mean serialized data stored as a file. Most likely the format to be YAML or maybe JSON (which suits best for micropython). In that way after reading the file a dict is created and search in dict works fast enough when in the same time takes not that much of ESP32 resources.