WAITAOF wad added in #11713, its return is an array.
But forget to handle WAITAOF in last_offset and last_numreplicas,
causing WAITAOF to return a WAIT like reply.
Tests was added to validate that case (both WAIT and WAITAOF).
This PR also refactored processClientsWaitingReplicas a bit for better
maintainability and readability.