Finalize locking mechanism

[adewes]

Currently several race conditions are possible in the bookSlot endpoint:

• The provider could delete the appointment slot the user is about to book during the booking procedure.
• Another user could book the same appointment slot.

While the likelihood of this happening is negligible for a small number of users it can become larger as the number of concurrent users grows. We should therefore use the Redis WATCH mechanism to ensure that no race conditions occur during the booking process.

[adewes]

Thanks for implementing this @johannwagner, I'll finalize the locking the coming days.