• Drop support for MuxedAccounts strkeys (spec'ed in SEP23).
  SEP23 is still a draft and we don't want to encourage storing strkeys which may not be definite.
• Replace SequenceProvider implementation with one which queries the Horizon DB for sequence numbers instead of the Stellar Core DB.
• Use the Horizon DB instead of Horizon's in memory order book graph to query orderbook details for the /order_book endpoint.
• Remove JSON variant of GET /metrics, both in the server and client code. It's using Prometheus format by default now.
• Decreased memory usage of initial state ingestion stage and state verifier (#2618).
• Remove --exp-ingest-in-memory-only Horizon flag. The in memory order book graph which powers the path finding endpoints is now updated using the Horizon DB instead of directly via ingestion (#2630).