fix: allow global leader lock context merge to be canceled (#16603)

Global leader lock was first designated to only acquired once
until the node is killed. However, currently, the code acquires
it repeatedly during the lifetime of the server, now there is a
goroutine leak.

cmd/shared-lock.go

@@ -63,6 +63,8 @@ func mergeContext(ctx1, ctx2 context.Context) (context.Context, context.CancelFu
 		select {
 		case <-ctx1.Done():
 		case <-ctx2.Done():
+		// The lock acquirer decides to cancel, exit this goroutine
+		case <-ctx.Done():
 		}
 
 		cancel()