Optimise db performance of GetLedgerRange

[aditya1702]

What

Fixes #233

• Removes the LatestLedger key from meta table. Use GetLedgerRange function since it already contains information about the latest ledger.
• Make the GetLedgerRange faster by caching latestLedgerSeq and latestLedgerCloseTime. This leads to a single query to get the oldest ledger and its close time.

Why

The latestLedgerSeq key can be removed since we already get that info from ledger range function. The GetLedgerRange is used a lot now so it is important to make it faster by caching the values.

Known limitations

NA

[aditya1702]

@2opremio I will include caching the ledger range in another PR since I am testing things with it and this PR can be merged without it.

[2opremio]

Caching the latest ledger is pretty fundamental for performance, so I would rather wait to have caching in main.

[aditya1702]

Caching the latest ledger is pretty fundamental for performance, so I would rather wait to have caching in main.

@2opremio The latestLedger is still being cached - https://github.com/stellar/soroban-rpc/pull/256/files#diff-2b645f26f8eb502d1988698186482a5910b37509e606f968a3a60c927c397cfeL150. I was referencing the oldestLedger and the close times that comprise the range.

[2opremio]

Ah ok, then the PR description and title are outdated. Can you update them?

[2opremio]

Or do you mean that everything is cached and working except to the oldest ledger and the oldest ledger close time?

[2opremio]

OK, I've taken a look at the code

@2opremio The latestLedger is still being cached - https://github.com/stellar/soroban-rpc/pull/256/files#diff-2b645f26f8eb502d1988698186482a5910b37509e606f968a3a60c927c397cfeL150. I was referencing the oldestLedger and the close times that comprise the range.

As I recall, this places it into the downstream cache, but doesn't really use the cache (as you can see in the code the code doesn't check the cache for the value it just writes to it). We need to be careful about using GetLedgerRange() as opposed to GetLatestLedger() here because it's more expensive.

One option is to actually read from the cache there but I think there were some subtleties preventing from doing so. Can you take a look?

[2opremio]

I am missing a migration to remove the entry from the meta table.