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-whole.go

@@ -17,6 +17,7 @@
 package cmd
 
 import (
+	"context"
 	"fmt"
 	"hash"
 	"io"
@@ -34,7 +35,7 @@ type wholeBitrotWriter struct {
 }
 
 func (b *wholeBitrotWriter) Write(p []byte) (int, error) {
-	err := b.disk.AppendFile(b.volume, b.filePath, p)
+	err := b.disk.AppendFile(context.TODO(), b.volume, b.filePath, p)
 	if err != nil {
 		logger.LogIf(GlobalContext, fmt.Errorf("Disk: %s returned %w", b.disk, err))
 		return 0, err
@@ -69,7 +70,7 @@ type wholeBitrotReader struct {
 func (b *wholeBitrotReader) ReadAt(buf []byte, offset int64) (n int, err error) {
 	if b.buf == nil {
 		b.buf = make([]byte, b.tillOffset-offset)
-		if _, err := b.disk.ReadFile(b.volume, b.filePath, offset, b.buf, b.verifier); err != nil {
+		if _, err := b.disk.ReadFile(context.TODO(), b.volume, b.filePath, offset, b.buf, b.verifier); err != nil {
 			logger.LogIf(GlobalContext, fmt.Errorf("Disk: %s returned %w", b.disk, err))
 			return 0, err
 		}