Fix: Remove slot mapping at bankd when client disconnects
If a client disconnects before removing the slot mapping, the worker must undefine its 'bank_id' and 'slot_nr' and must change its state to 'UNMAPPED'.
send_signal_to_worker() searches for a worker that has a given bank and slot. If a client re-connects to a different worker and if the bank and slot would be still assigned to the old worker, the old worker could receive signals when assigning or removing slot mapping. The new worker would not be mapped.
