Update FST GC for 'eos evict'

As discussed in #278 (closed), we have added a new command eos evict, more powerful than eos stagerrm and intended to replace it.

The FST garbage collector is one of the tools that will need to be changed to use eos evict.

This change must take into account that some deployments/components of EOS may still not have been updated to support eos evict, so we must take this into account.