Refactor redis keys and store messages in protobuf

Changes:
- Task messages are stored under "asynq:{<qname>}:t:<task_id>" key in redis, value is a HASH type and message are stored under "msg" key in the hash. The hash also stores "deadline", "timeout".
- Redis LIST and ZSET stores task message IDs
- Task messages are serialized using protocol buffer

forwarder_test.go

@@ -130,7 +130,7 @@ func TestForwarder(t *testing.T) {
 		for qname, want := range tc.wantPending {
 			gotPending := h.GetPendingMessages(t, r, qname)
 			if diff := cmp.Diff(want, gotPending, h.SortMsgOpt); diff != "" {
-				t.Errorf("mismatch found in %q after running forwarder: (-want, +got)\n%s", base.QueueKey(qname), diff)
+				t.Errorf("mismatch found in %q after running forwarder: (-want, +got)\n%s", base.PendingKey(qname), diff)
 			}
 		}
 	}