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-metadata-utils.go

@@ -126,7 +126,7 @@ func readAllFileInfo(ctx context.Context, disks []StorageAPI, bucket, object, ve
 			if disks[index] == nil {
 				return errDiskNotFound
 			}
-			metadataArray[index], err = disks[index].ReadVersion(bucket, object, versionID)
+			metadataArray[index], err = disks[index].ReadVersion(ctx, bucket, object, versionID)
 			if err != nil {
 				if err != errFileNotFound && err != errVolumeNotFound && err != errFileVersionNotFound {
 					logger.GetReqInfo(ctx).AppendTags("disk", disks[index].String())