Add flags to enable ingestion load test ledger backend in rpc

[tamirms]

We want to add support for running ingestion load tests in rpc.

#429 is a WIP PR which adds the flags to rpc to run ingestion load tests. However, it is still missing functionality for rolling back state changes to the rpc DB that were incurred during the load tests. For ideas on how to implement that functionality you can take a look at how it's done in horizon (see stellar/go#5794 ).