fix: various optimizations, idiomatic changes (#9179)

- acquire since leader lock for all background operations
  - healing, crawling and applying lifecycle policies.

- simplify lifecyle to avoid network calls, which was a
  bug in implementation - we should hold a leader and
  do everything from there, we have access to entire
  name space.

- make listing, walking not interfere by slowing itself
  down like the crawler.

- effectively use global context everywhere to ensure
  proper shutdown, in cache, lifecycle, healing

- don't read `format.json` for prometheus metrics in
  StorageInfo() call.

cmd/disk-cache-backend.go

@@ -229,7 +229,7 @@ func (c *diskCache) toClear() uint64 {
 }
 
 // Purge cache entries that were not accessed.
-func (c *diskCache) purge(ctx context.Context, doneCh <-chan struct{}) {
+func (c *diskCache) purge(ctx context.Context) {
 	if c.diskUsageLow() {
 		return
 	}