Robust system for dealing with time of events

[RockinRoel]

Currently, i++ relies on having the readers and processors having their clock set the same (more or less, within one second or so is good enough). This is not always very robust, especially when working
with Raspberry Pis, which do not have a clock battery.

We should come up with some sort of system that makes i++ work regardless of these time differences. Note that this is a very delicate matter. It could complicate the implementation of the reader, and make it harder to implement alternative reader implementations. Also, what if the date changes on the reader?

The RFID readers have their own clock, which is battery-backed, but maybe it's not a good idea to have the system rely on this, especially if we want to introduce other reader technologies, such as BLE.