Define custom cereal override for ConfigUpgradeSetKey and use it

[sisuresh]

Description

Resolves #4246

The new output now looks like -

[Herder INFO] Armed with network upgrades: upgradetime=2000-07-21T22:04:00Z, {
    "configupgradesetkey": {
        "contractID": "CDEWOG5BY6RL6HPKAO5GUPMZDEW2Y22V3J7PHI3WZN7HASTWGTMI6ITO",
        "contentHash": "7fa751106b2a7988dea3af733ca86484f1581f97c2ec9553bb36d402c612847d"
    }
}

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[dmkozh]

r+ f3ffbbe