storage duration/agreement between hosts and users

[floalex]

To further incentivize more hosts to share their storage in the network, having a storage duration agreement before uploading will be more fair to the hosts.

For example, our system can suggest a default storage duration will be 3 months for each file. Before the storage duration expiring, system will notify users and then users need to decide whether they would like to extend or not. If users want to extend, we need to verify if users' wallet and subtract the payment on the first day of the extension.