add '.healing.bin' for tracking currently healing disk (#10573)

add a hint on the disk to allow for tracking fresh disk
being healed, to allow for restartable heals, and also
use this as a way to track and remove disks.

There are more pending changes where we should move
all the disk formatting logic to backend drives, this
PR doesn't deal with this refactor instead makes it
easier to track healing in the future.

cmd/admin-heal-ops.go

@@ -85,7 +85,7 @@ type healSequenceStatus struct {
 
 // structure to hold state of all heal sequences in server memory
 type allHealState struct {
-	sync.Mutex
+	sync.RWMutex
 
 	// map of heal path to heal sequence
 	healSeqMap     map[string]*healSequence
@@ -105,21 +105,21 @@ func newHealState() *allHealState {
 }
 
 func (ahs *allHealState) healDriveCount() int {
-	ahs.Lock()
-	defer ahs.Unlock()
+	ahs.RLock()
+	defer ahs.RUnlock()
 
 	return len(ahs.healLocalDisks)
 }
 
 func (ahs *allHealState) getHealLocalDisks() Endpoints {
-	ahs.Lock()
-	defer ahs.Unlock()
+	ahs.RLock()
+	defer ahs.RUnlock()
 
-	var healLocalDisks Endpoints
+	var endpoints Endpoints
 	for ep := range ahs.healLocalDisks {
-		healLocalDisks = append(healLocalDisks, ep)
+		endpoints = append(endpoints, ep)
 	}
-	return healLocalDisks
+	return endpoints
 }
 
 func (ahs *allHealState) popHealLocalDisks(healLocalDisks ...Endpoint) {