Don't reapply norms already applied in the same session

[tobias]

This makes norm application faster by not repeatedly applying the same norms. This can be slow when there are many norms with a deep dependency tree.

In order to implement this, the structure was changed significantly:

• the reduction accumulator is no longer passed all the way down to reduce-txes
• error-handling is now top-level

We are currently using a fork of conformity at Shortcut that includes this change because we have a fairly complicated norm dependency tree. Before this change, a full conformity run against a fresh db would take 90 seconds. With this change, it now takes 1.8 seconds.

I'm offering it upstream in case it is useful for others.