Fix: Prevent race conditions when accessing slotmap in bankd
All threads (main and workers) can use slotmap_by_bank() and slotmap_by_client() to get a mapping entry from the slot map. While working with this entry (reading values), it could be deleted by a different thread.
For example, if a client connects to a worker, the worker will check if there is an existing mapping between a reader and this client. If it exists, slotmap_by_client() will return a pointer. If the mapping is deleted at this time by the server, the worker uses a pointer to a mapping entry that has just been freed.
