Refactor the meaning of pool.exists()

[abarmat]

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Could this be pool.exists()? Or, should the definition of pool.exists change? (I noticed now it's possible that a pool went through pool.init but its effective allocated state is still zero, so not sure how to interpret it or what's the best way to account for it)

Originally posted by @pcarranzav in https://github.com/graphprotocol/contracts/pull/549#discussion_r908606458