Perf testing for many-to-many

[ajcvickers]

• Query
• Insert/Update/Delete
• DetectChanges

[AndriySvyryd]

Detect changes with implicit/explicit join entity type

[ErikEJ]

Are you creating indexes "both ways" (on Left, Right AND on Right, Left) ? - I find that they have always been useful

[ajcvickers]

Part of #22588