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_test.go

@@ -134,7 +134,7 @@ func newErasureTestSetup(dataBlocks int, parityBlocks int, blockSize int64) (*er
 		if err != nil {
 			return nil, err
 		}
-		err = disks[i].MakeVol("testbucket")
+		err = disks[i].MakeVol(context.Background(), "testbucket")
 		if err != nil {
 			return nil, err
 		}