Implement fast TXMeta

[ire-and-curses]

This is the meta issue for the fast TXMeta project.

The goal is to decouple Horizon from the stellar-core database, remove the need for stellar-core to maintain txmeta internally, and allow Horizon to come up to date with historical ledger information quickly and easily.

"Offline mode" means the bulk replay mode where a captive stellar-core with in-memory ledger processing is used to generate historical txmeta very quickly.

"Online mode" means the current-ledger trickle mode where a captive stellar-core provides on-going ledger updates in real-time.

"FSC" means "fast stellar-core", a stellar-core instance running in-memory solely for the purpose of producing txmeta.

Offline mode

• Clean up single threaded prototype implementation (Fast txmeta: clean up single-threaded prototype #2551)
• Implement and profile parallel ingestion (Fast txmeta: implement and profile parallel ingestion #2552)
• Produce a benchmark that demonstrates that Horizon can complete a full ingestion in under 24 hours on a x1.32xlarge AWS machine.

Online mode

• Discuss whether we need online mode to run in-memory
• Identify any needed work on stellar-core side and raise issues
• Update interface to communicate bulk (offline) vs trickle (online) mode
• Update prototype to support online mode
• Produce a test demonstrating code can catch up offline and then switch to online mode
• Confirm crash/restart works reliably during online mode (no corruption etc)
• Identify how to implement transaction submission

Clustering

• Remove requirement for front-end Horizon nodes to directly ingest to memory
• Implement support for multiple online Horizon-FSC instances for redundancy

Packaging

• Determine how to optimally package and distribute Horizon-FSC for ease of use

Cleanup

• Remove Horizon accesses to stellar-core DB during ingestion
• Remove stellar-core queries made from the txsub package for the case where Horizon is configured to ignore failed transactions (implement a short lived failed transactions cache in ingestion instead?) and transaction status is requested.
• Clean up and remove all cursors and any vestigial code that accesses the stellar-core database.

Stretch goals (nice to have)

• Implement XDR contract changes to allow partial ledger processing.
• Fall back to sqlite for more modest hardware / very large ledgers
• Confirm Horizon-FSC runs on Windows (Docker container sufficient)

[bartekn]

Closing because all the items here are done, in separate issues or will be added later during a code cleanup stage.

[ire-and-curses]

So. Cool.