cls/queue: always set member variables in ctor

cls/queue: always set member variables in ctor

this should address the test failures like

```
/tmp/typ-WWFeFl6yK /tmp/typ-sMfwoaGMU differ: byte 24, line 1
**** cls_2pc_reservation test 2 binary reencode check failed ****
   ceph-dencoder type cls_2pc_reservation select_test 2 encode export /tmp/typ-WWFeFl6yK
   ceph-dencoder type cls_2pc_reservation select_test 2 encode decode encode export /tmp/typ-sMfwoaGMU
2c2
< 00000010  00 00 00 00 00 00 c0 c6  92 10                    |..........|
---
> 00000010  00 00 00 00 00 00 c0 e6  cd 53                    |.........S|
```

where we
1. encode the 2nd sample created by `generate_test_instances()`,
2. encode, decode, and encode again, the same sample

and compare the encoded blobs.

but if we fail to set any of the fields in `cls_2pc_reservation`,
we are at the mercy of the random bits on stack/heap.

in this change, all bits are initialized.

the flaky test was introduced by 1d7cabf3

Signed-off-by: Kefu Chai <tchaikov@gmail.com>