R1:4d86d51125fe

Prepare TransactionCommentQuery for extension

Summary:
Ref T2009. Ref T1460. The way Diffusion and Differential load inlines is horrible garbage right now:

- Differential does an ad-hoc query to get the PHIDs, then does a real load to policy check.
- Diffusion completely fakes things. In practice this is not a policy violation, but it's dangerous.

Make TransactionCommentQuery extensible so we can subclass it and get the query building correctly in the right Query layer.

Specifically, the Diffusion…