Provide configuration guidelines for BufferedStorageBackend settings

[urvisavla]

What problem does your feature solve?

In #4911, we added an option for Horizon to reingest historical ledgers from the datastore using BufferedStorageBackend. BufferedStorageBackend has a BufferSize that specifies the number of ledgers to hold in memory and a NumWorkers to specify the number of parallel download workers. We need to run tests and benchmark to identify the optimal configuration values.

What would you like to see?

Configuration guidelines for BufferedStorageBackend.

What alternatives are there?

Let users find the best settings through experimentation.