Walk the merged document.
For operations which are marked as no-bwc, reclassify breaking diffs as follows

• for each such diffs, create a copy, with type changed to risky,
• remove original diff from DIFF_META_KEY symbol
• add new diff to DIFF_META_KEY symbol instead
• the same diff could occur several times in the tree and each occurens should be replaced with the same instance of risky diff, so need to keep a map of original diffs to reclassified diffs and not create new if one was already created

Subtrees for operations which are not marked as no-bwc should be skipped.
Should reclassify before diff aggregation.

Add new tests:

• two operations (bwc and no-bwc) use the same schema, breaking change happens in the schema, check that there's both breaking and risky diff for corresponding operations
• the same schema used several times in a no-bwc operation, check that the same risky diff is used in each occurrence