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-common.go

@@ -77,7 +77,7 @@ func (er erasureObjects) isObject(ctx context.Context, bucket, prefix string) (o
 				return errDiskNotFound
 			}
 			// Check if 'prefix' is an object on this 'disk', else continue the check the next disk
-			return storageDisks[index].CheckFile(bucket, prefix)
+			return storageDisks[index].CheckFile(ctx, bucket, prefix)
 		}, index)
 	}