Add storage layer contexts (#10321)

Add context to all (non-trivial) calls to the storage layer. 

Contexts are propagated through the REST client.

- `context.TODO()` is left in place for the places where it needs to be added to the caller.
- `endWalkCh` could probably be removed from the walkers, but no changes so far.

The "dangerous" part is that now a caller disconnecting *will* propagate down,  so a 
"delete" operation will now be interrupted. In some cases we might want to disconnect 
this functionality so the operation completes if it has started, leaving the system in a cleaner state.

cmd/erasure-healing-common_test.go

@@ -210,7 +210,7 @@ func TestListOnlineDisks(t *testing.T) {
 				// and check if that disk
 				// appears in outDatedDisks.
 				tamperedIndex = index
-				dErr := erasureDisks[index].DeleteFile(bucket, pathJoin(object, fi.DataDir, "part.1"))
+				dErr := erasureDisks[index].DeleteFile(context.Background(), bucket, pathJoin(object, fi.DataDir, "part.1"))
 				if dErr != nil {
 					t.Fatalf("Test %d: Failed to delete %s - %v", i+1,
 						filepath.Join(object, "part.1"), dErr)