pull: Rework delta superblock fetches to be async

pull: Rework delta superblock fetches to be async

For the pending libcurl port, the backend is a bit more
sensitive to the main context setup.  The delta superblock
fetch here is a synchronous request in the section that's
supposed to be async.

Now, libsoup definitely supports mixing sync and async requests, but it wasn't
hard to help the libcurl port here by making this one async. Now fetchers are
either sync or async.

Closes: #636
Approved by: jlebon