Fix sqlite backend KeepAlive revision mismatch (#66930)

Ensures the backend table and events table use the same revision when
updating items during KeepAlive operations. Previously a revision was
generated for _both_ tables causing watchers and Get calls to observe
different revisions.

The backend test suite was updated to validate revisions in the event
stream match revisions in the backend.

Resolves https://github.com/gravitational/pressure-washing/issues/139.

lib/backend/lite/lite.go

@@ -781,7 +781,7 @@ func (l *Backend) KeepAlive(ctx context.Context, lease backend.Lease, expires ti
 		}
 		defer stmt.Close()
 
-		result, err := stmt.ExecContext(ctx, expires.UTC(), id(now), backend.CreateRevision(), lease.Key.String())
+		result, err := stmt.ExecContext(ctx, expires.UTC(), id(now), item.Revision, lease.Key.String())
 		if err != nil {
 			return trace.Wrap(err)
 		}

lib/backend/test/suite.go

@@ -760,6 +760,10 @@ func testKeepAlive(t *testing.T, newBackend Constructor) {
 	require.Equal(t, bigValue[:], event.Item.Value)
 	require.WithinDuration(t, expiresAt, event.Item.Expires, 2*time.Second)
 
+	storedItem, err := uut.Get(ctx, item.Key)
+	require.NoError(t, err)
+	require.Equal(t, storedItem.Revision, event.Item.Revision)
+
 	// move the current slightly forward, but still *before* the item's
 	// expiry time
 	clock.Advance(2 * time.Second)
@@ -776,6 +780,10 @@ func testKeepAlive(t *testing.T, newBackend Constructor) {
 	require.Equal(t, bigValue[:], event.Item.Value)
 	require.WithinDuration(t, updatedAt, event.Item.Expires, 2*time.Second)
 
+	storedItem, err = uut.Get(ctx, item.Key)
+	require.NoError(t, err)
+	require.Equal(t, storedItem.Revision, event.Item.Revision)
+
 	err = uut.Delete(t.Context(), item.Key)
 	require.NoError(t, err)