File sharing feature
Created by: tlightsky
an incorrect rebase close the pr before: https://github.com/NebulousLabs/Sia/pull/1978
now i rebase all the code to newest master and make a pr again
some problem still unsolved though:
- how many contracts should be formed(if a file is already avaiable,should we make more to download it more faster?)
- how long the contract should be(same with allowance? what to do if no allowance, default a week?)
- should we renew the
download only contract
- should we use the money directly from wallet(without confirm from user?)