R1:167f06d3eb70

Label transaction groups with a "group ID" so Herald can reconstruct them faithfully

Summary:
Ref T13283. See PHI1202. See D20519. When we apply a group of transactions, label all of them with the same "group ID".

This allows other things, notably Herald, to figure out which transactions applied together in a faithful way rather than by guessing, even though the guess was probably pretty good most of the time.

Also expose this to `transaction.search` in case callers want to do something similar. They…