soroban-rpc: optimize simulateTransaction by querying ledger entry/expiration at the same time

[tsachiherman]

Background

In stellar/stellar-cli#916, we combined the database query to retrieve the ledger key + expiration key at the same time ( for code/data entries ). The above PR also updated the RPC calls to return these.

However, the internal usage of simulateTransaction was never taking advantage of that.

Remaining work

The SnapshotSourceGet should be able to return the expiration round as well. That would allow us to make a single query from rust->go instead of two.

[2opremio]

This is done by the rs-soroban-simulation library now