Context

On non-primaries, verify delete oplog entries: before deleting, recompute the pre-image's hash and size and compare against the primary's h.

On a mismatch, fassert with a stack crash until we have different modes for handling a mismatch.

Runs only on non-primaries (OplogApplication::Mode::kSecondary), with the feature flag on, for supported collections, and only when h is present. We may want to add a helper for checking these conditions.

Acceptance criteria

• Before deleting, the in-hand pre-image is hashed (SHA-256) and its size computed, then both compared against h.
• On mismatch, fassert and crash.
• Skipped on the primary, when the feature flag is off, on unsupported collections, and when m.h is absent.
• No update/insert handling (done in SERVER-128585 and SERVER-128516 respectively)
• Unit-test!
  • Round-trip / happy path: The recomputed h and size match the primary's h and no mismatch is reported.
  • Mismatch detected: when h diverges, the comparison reports a mismatch.
  • Without the feature flag or without h, we don't perform the check.
  • Non-steady-state modes (initial sync, recovering) neither generate nor compare h.
  • Unsupported collections, and entries without h, are skipped.
  • Any helper functions that we use that can be unit tested are also unit-tested.