R1:814e6d2de927

Add more type checking to transactions queued by Herald

Summary:
See PHI1096. Depends on D20213. An install is reporting a hard-to-reproduce issue where a non-transaction gets queued by Herald somehow. This might be in third-party code.

Sprinkle the relevant parts of the code with `final` and type checking to try to catch the problem before it causes a fatal we can't pull a stack trace out of.

Test Plan: Poked around locally (e.g., edited revisions to cause Herald to trigger), but hard to know if this…