allow to flag a storage as 'shared', which means that the same underlying storage is shared among the peers

this instructs shardcache to fallback to the storage even when a peer is not the owner of a key and it wasn't able
to obtain the value from the responsible peer (because of dead or anyway unreachable)

This allows to use , for example, a shared database or a shared filesystem as actual storage , accessible to all the peers.
If all peers are healthy , each will end up accessing the db only when fetching the owned keys . But in case one of the peers
is faulty, data will still be accessible by all other peers and can fallback to accessing the storage if communication with the faulty peer
fails