To help track down potential correctness issues with RocksDB and Pebble as well as our MVCC implementations on top of those, a new engine type pebble-rocksdb or pebble+rocksdb should be created, that spins up instances of both RocksDB and Pebble (in subdirectories from the main provided data directory), writes to both for any write-related operations, and compares the two in all read paths, panicking if there's a difference.

Should be selectable as an engine type with the COCKROACH_STORAGE_ENGINE env variable, and the --storage-engine flag, but it's okay if it's not documented since uses of this engine would be limited to testing.

Rough implementation plan:

• Implement TeeingEngine (with read-only variant/flag), TeeingBatch (with distinct variant/flag), TeeingIterator (with ability to work on batches), implementing the appropriate interfaces
• Add new EngineType and new engine constructor that takes in config for both RocksDB and Pebble
• Wire up config logic in server/config.go
• Anything else that needs to be done (eg. TestServer updates) to support this new engine type.