Fix potential data race in the PC/SC-based reader tracker

[emaxx-google]

We have the functionality that tracks the list of currently available smart card readers based on the information exposed by the PC/SC API.
This implementation has a potential data race that it can miss the changes if they happen between the completion of one PC/SC call and the beginning of the next one. In that case, the change would only be recognized after some time (60 seconds at worst with the current implementation). This bug tracks the fix of this data race.

After this data race gets fixed, we could actually make a small performance improvement by avoiding the need to make a periodic PC/SC call (every 60 seconds currently) from that code.