Microdescriptors: make clients use microdescriptors instead of router descriptors

The microdescriptor project ([legacy/trac#1748 (moved)]) will be done once clients finally use microdescriptors, instead of regular ones, to choose routers and build circuits.

This step needs to come last in the microdescriptor project; it's mainly going to be figuring out every fool thing we do that requires a routerinfo_t , and making sure that node_t can do it as well using microdescriptor_t.