Revamp bucket metadata healing (#7208)

Bucket metadata healing in the current code was executed multiple
times each time for a given set. Bucket metadata just like
objects are hashed in accordance with its name on any given set,
to allow hashing to play a role we should let the top level
code decide where to navigate.

Current code also had 3 bucket metadata files hardcoded, whereas
we should make it generic by listing and navigating the .minio.sys
to heal such objects.

We also had another bug where due to isObjectDangling changes
without pre-existing bucket metadata files, we were erroneously
reporting it as grey/corrupted objects.

This PR fixes all of the above items.

cmd/dummy-object-layer_test.go

@@ -119,7 +119,7 @@ func (api *DummyObjectLayer) HealFormat(ctx context.Context, dryRun bool) (item
 	return
 }
 
-func (api *DummyObjectLayer) HealBucket(ctx context.Context, bucket string, dryRun, remove bool) (items []madmin.HealResultItem, err error) {
+func (api *DummyObjectLayer) HealBucket(ctx context.Context, bucket string, dryRun, remove bool) (items madmin.HealResultItem, err error) {
 	return
 }