slapgrid: new option to garbage collect software that are no longer supplied
useful on theia1 of resilient theia, because in a scenario like:
- supply on theia0
- resilience replicate the supply on theia1
- destroy on theia0, software gets destoyed on theia0 and removed from slapos proxy databse.
- resilient just replicate the new proxy database without the row in software table
softwares are never destroyed in theia1.
Edited by Jérome Perrin