Issues:

• update in-place mutates a document
• in-place mutation may change the values of predicates evaluated via ixscans or filters. therefore we may return invalid results from query unless we broadcast the mutation.
• when a doc invalidation occurs, sort right now drops the doc. ditto for other stages.
• we don't want to drop docs whenever possible if in-place updates occur, therefore:
• may need to re-sort or evaluate full match expression at end.