Closes #3375. Restructures simulator initialization around factories that more or less behave like a hand-written dependency injection framework (as in [UnityContainer](http://unitycontainer.org/) for C#). The upshot is that this code is more capable of handling variations in initialization, e.g. block summary nodes vs core nodes. This also replaces `config.Describe` with `DescribeShim` in the executor packages to force the executor to stop using the outdated network config stuff, and consequentially this also updates the executors to pull the BVN names from the network variables instead of from the outdated network config.

Changelog: refactor