* Postgres fixes
* Create partial index for ledger_diffs, to avoid indexing the first
ledger's objects
* Don't write duplicate keys to successor table
* default to 4 markers when syncing cache
* remove isFirst from writeLedger interface
* Adds a cache for the most recently validated ledger state
* Replaces the keys table with successor table
* Adds support for ledger diffs in the database
* Removes BackendIndexer