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/bitrot_test.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"context"
 	"io"
 	"io/ioutil"
 	"log"
@@ -39,7 +40,7 @@ func testBitrotReaderWriterAlgo(t *testing.T, bitrotAlgo BitrotAlgorithm) {
 		t.Fatal(err)
 	}
 
-	disk.MakeVol(volume)
+	disk.MakeVol(context.Background(), volume)
 
 	writer := newBitrotWriter(disk, volume, filePath, 35, bitrotAlgo, 10)