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/global-heal.go

@@ -131,7 +131,7 @@ func healErasureSet(ctx context.Context, setIndex int, xlObj *erasureObjects, se
 				continue
 			}
 
-			entryCh, err := disk.WalkVersions(bucket.Name, "", "", true, ctx.Done())
+			entryCh, err := disk.WalkVersions(ctx, bucket.Name, "", "", true, ctx.Done())
 			if err != nil {
 				// Disk walk returned error, ignore it.
 				continue