GTCR Factory: Allow upgradable contract code.

[eccentricexit]

Currently the GTCR Factory simply keeps a copy of a version of the GTCR and deploys it. This has the downside that the version of the GTCR code to deploy is locked. New versions would require a new factory.

Instead, we can have the factory store the address of a GTCR instance. deploy() would then copy the code and deploy from that.

If the GTCR code is ever updated, we can update the address of the "blueprint" GTCR so that new versions can be deployed using the same factory.

[eccentricexit]

As discussed in call, this feature adds complexity (and thus harms security). Instead cloning contract, we can deploy a new factory and reference the previous one from the new contract.