Extension configuration handling: optimization opportunity

[oschaaf]

When the code for handling effective configuration in extensions is consolidated there might be an opportunity to optimize:

Each of our extensions independently read, validate and compute effective configuration based on request level configuration. This is good, because each on should be able to function independently. However, in the likely case that all of them are enabled, there should be a way to cache the results of the first extension and carry that across to extensions running after it for re-use, to eliminate redundant work.