Async checkupdates
Similar to what flatpak sources generator does: https://github.com/flatpak/flatpak-builder-tools/blob/52746ecfc130dcb5d509522b5e0c5a9defc43b76/node/flatpak_node_generator/progress.py#L72-L85. Async by appid
Tested to checkupdates all: https://gitlab.com/proletarius101/fdroidserver/-/jobs/2792440032
/cc @jspricke
Edited by proletarius101