Record path concatenator activity in state

[paul-butcher]

When faced with a path branch/leaf, the path concatenator looks up root/branch and returns root/branch/leaf. It modifies the collectionPath of the documents in situ (rather than by creating a whole new index to put it in, like other stages do).
This is partly because it is only expected to change a small number of records and it would be wasteful and awkward to make it into a whole new step. It is also because of the transitive nature of the operation.

However, this does mean that we lose some observability, compared to the other "real" stages. We do not know the state before the change, so if a bug occurs in the concatenator, and it changes something in a way that it should not, then we can't see that on the record.

It would be good to record this in the state section of the record. Information could include:

• path concatenation time
• original collectionPath
• any other records involved (this may be inappropriate, could be unwieldy)