Add Backup worker to handle PUTCHUNK messages

[fabiodrg]

The client requests the backup service. This splits the file in chunks each will be sent, one by one, to all the peers. Considering #17, the process of listening for STORED messages and ensuring the desired replication degree may take a while (5 retries, up to ~31 seconds). It makes sense to dispatch the service requests, like backup requests, to workers. I.e, the Peer splits the file in chunks as it is, it's a fast operation, but then launches a worker (thread) responsible for creating the message, await for responses and ensuring the replication degree. Then it's ready for incoming backup or other service requests.