lib/repo-pull: Support queuing delta superblock requests

lib/repo-pull: Support queuing delta superblock requests

Just like all the other requests made for delta parts and objects by the
pull code, use a queue for delta superblocks. Currently this doesn’t do
any prioritisation or retries after transient failures, but it could do
in future.

This means that delta superblocks are now subject to the parallel
request limit in the fetcher, which was a problem highlighted here:
https://github.com/ostreedev/ostree/pull/1453#discussion_r168321706.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #1600
Approved by: jlebon