Upgrade to use stellar/[email protected]

[leighmcculloch]

This PR is a draft and is likely incomplete.

I've taken a time-boxed pass at upgrading the stellar/go dependency from horizonclient-v3.0.0 to horizonclient-v8.0.0. I mostly just wanted to explore, understand, and validate assumptions about how Kelp could be upgraded, assuming that the upgrade path would not be smooth.

The intention was to understand, not necessarily deliver an upgrade.

I've opened this draft PR to have this change run on Kelp's CI, to see if at least from CI's perspective if this idea for how to make the upgrade happen, has substance or creates more problems than it solves.

The approach I've taken is to:

• Upgrade github.com/stellar/go.
• Preserve use of the old github.com/stellar/[email protected]/build by vendoring that package inside of kelp.
• Update references to types and functions within github.com/stellar/go/xdr to their new names, since some XDR types had their names changed.
• Make zero functional changes.

Maybe fix #741

[ddombrowsky]

It's a bit of a hack to put in a module from the old version of the SDK, but I think it works for now. In the long run it's cleaner than forking the SDK itself.

[nikhilsaraf]

LGTM, thanks @leighmcculloch !