Make read-only ledger state const

[marta-lokhova]

Improve access to ledger state, to better support parallelization changes in #4543

Note that management of SorobanNetworkConfig is still not great, as currently LM manages multiple versions of the config. Ideally, soroban network config lives inside of the state snapshot (either BucketList snapshot or LedgerTxn), but this was too tricky to implement at this time due to how network config is currently implemented. We may need to clean this up later.

This change also partially addresses #4318

[SirTyson]

I'm not a huge fan of the interface where we maintain two separate copies of SorobanNetworkConfigSettings at the LedgerManager level, as it sometimes unclear which method to use and can lead to tricky bugs (such as with background eviction). Given that the network settings are part of ledger state, I would still prefer threads to maintain a canonical snapshot and route all ledger based queries through that snapshot, including LedgerEntry, ledger header, and network setting accesses. That being said, I wanna get you unstuck, and am fine with this approach in the interim as we move towards more thread safety in general. To merge this I do think it would be good to clean up the new snapshot interface a bit, specifically with the maybeUpdateSnapshot issues I raised + atomic snapshot swaps.

[marta-lokhova]

@dmkozh @SirTyson thanks for taking a look! I believe your comments are addressed now.

[SirTyson]

Thanks for the changes, looking good! One minor bug and a bunch of small nitpicky cleanups.

[SirTyson]

LGTM