Context

We need to compute the document hash on the primary and populate the h field on the update oplog entry during steady-state replication. SERVER-128506 only produces h on the primary.

The hash is computed over the document affected by the op: the post-image for inserts and updates, and the pre-image for deletes. Use SHA-256 to start.

Acceptance criteria

 * Inserts on the primary set m.h when the feature flag is on with a hash of the document's post-image.
 * When the feature flag is off, m.h is not set on updates.
 * We unit-test the above two points.