MongoDB 8.0: Improving Performance, Avoiding Regressions

Introduced ExpressPlan as a new query plan stage to optimize simple lookups by bypassing the general query planner, building on the IDHACK concept. Modified the replication write acknowledgment to occur after journaling rather than in-memory application on secondaries, reducing primary latency and speeding up writes while maintaining durability. Implemented instruction counting using Linux perf_event_open syscall for detecting subtle performance regressions with a low tolerance.