Have frontend groups request the removal of unused glideins
When in the frontend there are no more jobs waiting to run in a group, the frontend could ask the factory to remove all leftover idle glideins.
There is already a tool to manually trigger the removal, remove_requested_glideins, but could be good to add the removal automatically in the frontend request cycle.
It would reduce waste when there is no need anymore for glideins.
Group1 glideins would not be left around for Group2 jobs to use, but if G2 jobs can run on G1 glideins (same credentials, same start expression), then these should be one group.
Glideins would still benefit from temporal multiplexing (multiple requests in the same group) if the new requests for jobs arrive before the glidein removal kicks in.
This should be a configurable feature, off by default for now.