Verifies accounts lt hash at startup

[brooksprumo]

Problem

Eventually, the accounts lt hash will not be recalculated at startup. Instead, we'll get the initial accounts lt hash from the snapshot (or equivalent). When that happens, we will still want to verify the accounts, and we'll want that to happen in the background, similar to today with the full accounts hash calculation. Currently there is not a way to verify the accounts lt hash that is suitable for running in the background.

Summary of Changes

Add a function to calculate the accounts lt hash from storages. Use that to verify the accounts lt hash at startup in the background.

Additional Testing

In addition to the tests in the PR, I ran this code against mnb on a dev box. This calculates the accounts lt hash from the index first, then verifies from storages second. This was successful (hashes matched). The verification step took approximately the same amount of time as the normal accounts hash verification (full + incremental).