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.go

@@ -175,9 +175,9 @@ func disksWithAllParts(ctx context.Context, onlineDisks []StorageAPI, partsMetad
 			// disk has a valid xl.meta but may not have all the
 			// parts. This is considered an outdated disk, since
 			// it needs healing too.
-			dataErrs[i] = onlineDisk.VerifyFile(bucket, object, partsMetadata[i])
+			dataErrs[i] = onlineDisk.VerifyFile(ctx, bucket, object, partsMetadata[i])
 		case madmin.HealNormalScan:
-			dataErrs[i] = onlineDisk.CheckParts(bucket, object, partsMetadata[i])
+			dataErrs[i] = onlineDisk.CheckParts(ctx, bucket, object, partsMetadata[i])
 		}
 
 		if dataErrs[i] == nil {