Pre-apply the bootstrapping ConfChange entries.

This eliminates the need to fake an ApplyConfChange call before Campaign
in tests.

Fixes #1856.

@xiang90 @yichengq 
Actually, a flag in the ConfChange message won't work since these bootstrap entries may also be replayed after RestartNode, in which case they should not be ignored. So if we want to treat these entries as special it will need to be based on transient state instead of something written to the log. Do we need to distinguish them, or is it OK if addNode simply ignores any nodes which are already present whether we're bootstrapping or not?