Pre-apply the bootstrapping ConfChange entries.

This eliminates the need to fake an ApplyConfChange call before Campaign
in tests.

Fixes #1856.

@xiang90 @yichengq 
Two reasons: 1) I need this to be in a public (non-test) function so I can use it from cockroach, and 2) if we don't put this before `go n.run(r)` there are concurrency concerns.