Fix issue with modifySorobanNetworkConfig

[SirTyson]

Description

Resolves an issue with modifySorobanNetworkConfig, where sometimes bucket levels would merge multiple times. Note meta has changed as some tests now close an additional ledger during modifySorobanNetworkConfig.

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

[marta-lokhova]

Can we force externalize + close ledger once with network config upgrades instead of what we're doing right now? I've been debugging some test failures in #4597, and it seems related, although I don't exactly understand what's going on in this flow. Right now we add batch and update snapshot, followed by an empty ledger close, which again adds batch and updates the snapshot. That feels quite error-prone. Why do we need to close an extra empty ledger?

[marta-lokhova]

took a while to figure out what's going on, but looks like ledger close meta in tests was broken because of double add batch (I updated the meta in 25e8023). When we manually add batch, we update snapshots in tests, but then when we actually close ledger (same ledger seq), we do not update snapshot because of this condition.