multi: add resolver reports to Checkpoint

To allow us to write the outcome of our resolver to disk, we add
optional resolver reports to the CheckPoint function. Variadic params
are used because some checkpoints may have no reports (when the resolver
is not yet complete) and some may have two (in the case of a two stage
resolution).

contractcourt/commit_sweep_resolver.go

@@ -277,7 +277,7 @@ func (c *commitSweepResolver) Resolve() (ContractResolver, error) {
 	c.reportLock.Unlock()
 
 	c.resolved = true
-	return nil, c.Checkpoint(c)
+	return nil, c.Checkpoint(c, nil)
 }
 
 // Stop signals the resolver to cancel any current resolution processes, and