misc cleanup while reviewing chans/goroutines (#1375)

- renamed snapshot_rollback.go and snapshot_rollback_test.go to
  rollback.go and rollback_test.go, the snapshot_ prefix
  should be limited to code that relates directly to either
  an index snapshot or segment snapshot.  rollback can be seen
  as operating on the index as a whole.

- renamed mainLoop to introducerLoop, consistent with
  persisterLoop and mergerLoop

- remove snapshotReversion this is no longer used

- remove type uint64Descending and related methods, no longer used

index/scorch/introducer.go

@@ -45,13 +45,7 @@ type epochWatcher struct {
 	notifyCh notificationChan
 }
 
-type snapshotReversion struct {
-	snapshot  *IndexSnapshot
-	applied   chan error
-	persisted chan error
-}
-
-func (s *Scorch) mainLoop() {
+func (s *Scorch) introducerLoop() {
 	var epochWatchers []*epochWatcher
 OUTER:
 	for {

index/scorch/persister.go

@@ -780,12 +780,6 @@ func (s *Scorch) loadSegment(segmentBucket *bolt.Bucket) (*SegmentSnapshot, erro
 	return rv, nil
 }
 
-type uint64Descending []uint64
-
-func (p uint64Descending) Len() int           { return len(p) }
-func (p uint64Descending) Less(i, j int) bool { return p[i] > p[j] }
-func (p uint64Descending) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
-
 func (s *Scorch) removeOldData() {
 	removed, err := s.removeOldBoltSnapshots()
 	if err != nil {

index/scorch/scorch.go

@@ -181,7 +181,7 @@ func (s *Scorch) Open() error {
 	}
 
 	s.asyncTasks.Add(1)
-	go s.mainLoop()
+	go s.introducerLoop()
 
 	if !s.readOnly && s.path != "" {
 		s.asyncTasks.Add(1)