Switch to BT SPP instead of BLE

[Xasin]

As the ESP BT Library decided to be a pain to work with, and sadly does not provide Low Energy (due to a APB Max Freq Lock), it does not provide any benefits over classic BT in SPP mode.

As such, a DIY protocol should be created to utilize BT SPP in regular "burst" exchanges, to save energy and be easy to use. The following criteria should be met:

• [ ] "EOT" Signal code when all values are up-to-date
• [x] Sending of arbitrary data with arbitrary length, grouped by a 16-bit packet ID. Maybe Base64 encoding for better security and newline separation?
• [x] "Overwrite" and "Stream" characteristics, which either provide a single, "current" value, or a stream of data (battery mV versus command stream)
• [x] Seamless use of "write" functions regardless of connection status (i.e. immediate update push or buffer until reconnected)

[Xasin]

Only the EOT signal and auto-reconnect remain now - looking good!