Repo Size Constraints

It is a common problem already that the space of go-ipfs grows unbounded. We want to have the ability to limit the growth.

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Complicating factors:
- pinned vs unpinned objects
- capacity should be configurable
- capacity should incorporate all repo contents
- adding (pinning) content beyond capacity

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I've been thinking of a setup with two thresholds:
- gc threshold: if after this limit, all commands trigger a `repo gc`.
- local storage capacity: if adding an object to repo would increase capacity, command errors. (i.e. never exceed capacity)