Fix curle partial file on downloading index

[gportay]

Hi, This is a proposal to fix the issue mentioned in the second commit. This solution as a drawnback There is an alternative solution. It consists in fixing the issue in casync process by updating the state machine to download the file after seeding. In my solution, the size of the index is limited to CA_PROTOCOL_SIZE_MAX (1610241024). Is it enough for an index file? @poettering, @keszybz, what do you think?

[keszybz]

Sorry for letting this stall. I'll go ahead an merge this, since it's a solution to the issue, even if imperfect.