refactor healing to remove certain structs (#13079)

- remove sourceCh usage from healing
  we already have tasks and resp channel

- use read locks to lookup globalHealConfig

- fix healing resolver to pick candidates quickly
  that need healing, without this resolver was
  unexpectedly skipping.

cmd/metacache-entries.go

@@ -163,6 +163,10 @@ func (e *metaCacheEntry) fileInfo(bucket string) (*FileInfo, error) {
 		}, nil
 	}
 	if e.cached == nil {
+		if len(e.metadata) == 0 {
+			// only happens if the entry is not found.
+			return nil, errFileNotFound
+		}
 		fi, err := getFileInfo(e.metadata, bucket, e.name, "", false)
 		if err != nil {
 			return nil, err
@@ -308,15 +312,21 @@ func (m metaCacheEntries) resolve(r *metadataResolutionParams) (selected *metaCa
 		sort.Slice(r.candidates, func(i, j int) bool {
 			return r.candidates[i].n > r.candidates[j].n
 		})
+
 		// Check if we have enough.
 		if r.candidates[0].n < r.objQuorum {
 			return nil, false
 		}
+
 		if r.candidates[0].n > r.candidates[1].n {
-			return r.candidates[0].e, true
+			ok := r.candidates[0].e != nil && r.candidates[0].e.name != ""
+			return r.candidates[0].e, ok
 		}
+
+		e := resolveEntries(r.candidates[0].e, r.candidates[1].e, r.bucket)
 		// Tie between two, resolve using modtime+versions.
-		return resolveEntries(r.candidates[0].e, r.candidates[1].e, r.bucket), true
+		ok := e != nil && e.name != ""
+		return e, ok
 	}
 }