Stop storing state

[2opremio]

More context: #353

Protocol 23 comes with complicated state archival changes for which we will be delegating all state management to Captive Core. In particular, all the state querying will happen through HTTP calls to Captive Core.

As a side effect, we no longer need to manage state (i.e. ledger entry values). Thus we need to remove:

1. All state book-keeping code from the ingestion code.
2. The state table (ledger_entries) from the database. This will require a SQL migration. We should confirm the migration works with a full DB using pubnet to ensure there are no issues.
3. The (unreleased) --stellar-captive-core-http-query-port flag (currently defaulting to 0) since Captive Core's query port needs to be enabled by default for RPC to work. Alternatively, we can leave the flag but make it default to a non-zero value (note that even if we remove the flag, the value is still configurable by the captive core config file).