#3983 Latest upgrade command

[Anonymous]

Originally created by: sisuresh

Description

Resolves parts of https://github.com/stellar/stellar-core/issues/3822.

1. The get-settings-upgrade-txs command line option will print out transactions to submit to the network to set up a settings upgrade.
2. The dumpproposedsettings HTTP endpoint will print out the ConfigUpgradeSet for a given ConfigUpgradeSetKey.

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