Use a single DB transaction within getTransactions

[aditya1702]

Currently, both these methods involve a call to GetLedgerRange to get the current RPC range and then subsequent calls to the DB to get different ledger close meta. This introduces a race condition where ingestion can happen between these 2 calls and change the ledger range snapshot in the db.

To mitigate this, we want to use a single DB transaction throughout the entire lifecycle of the request.