Replace getLedgerEntries() with a proxy to Core's /getledgerentry.

[Shaptic]

Protocol 22: Replacing Endpoints

Epic: stellar/go#5433
References: Platform's issues for Protocol 22: projects/37#31

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Stellar Core will have support a high-performance HTTP endpoint for retrieving ledger entries as part of their v22 in stellar/stellar-core#4350. Once stellar/go#5426 is complete, we can completely remove the implementation of getLedgerEntries and its database backing and replace it with a proxy to the Core client.

Acceptance Criteria

The database tables are dropped.
The implementation is a proxy to Core.
The returned JSON schema stays the same.
The performance is still acceptable.

[2opremio]

@Shaptic did you start working on this? Otherwise I will take it

[Shaptic]

@2opremio this is blocked on Core actually providing this endpoint, but you can take it up if you want to assume the existence of Core's endpoint. I'm working on client support in stellar/go#5428 but again, no real Core yet.

Also note that we will still need entry ingestion + db snapshotting until P23.