Refactor insert/materialize node schemas.

Another subtlety here is the source of the non-inline memory for string/varbinary default values.
Our choices are:
   The persistent data pool -- as referenced by the final copies of the tuples.
    The thread-local temp pool -- as reclaimed wholesale at the end of the execution.
I don't off-hand have a reason to prefer one vs. the other. The latter is only an option when/because the strings are allocated and used (copied to persistent storage) temporarily during the lifetime of the "current execution". It is a "non-issue" in terms of runtime resource usage because it is called once per string value during the current execution.