Built-in GC configuration to run periodically

[hacdias]

Problem: Right now, the GC needs to be triggered externally, which is far from the best situation.

Solution: we could periodically run the GC in Rainbow by default, that would delete a certain number of bytes based on the disk space, if possible.

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@ns4plabs I heard you may be working on this. But if you aren't, I can take a stab at it.