fix: Simplify provisionerd job acquire (#158)

This uses a simple channel to detect whether a
job is running or not, and moves all cancels
to be in goroutines.

coderd/provisioners.go

@@ -12,7 +12,7 @@ type ProvisionerJobStatus string
 
 // Completed returns whether the job is still processing.
 func (p ProvisionerJobStatus) Completed() bool {
-	return p == ProvisionerJobStatusSucceeded || p == ProvisionerJobStatusFailed
+	return p == ProvisionerJobStatusSucceeded || p == ProvisionerJobStatusFailed || p == ProvisionerJobStatusCancelled
 }
 
 const (