memcached mismatched return value and result code

### HHVM Version

HipHop VM 3.23.4 (rel)
Compiler: tags/HHVM-3.23.4-0-g90c8bcc5927c21d506a4a4f26b140cdb534e6bed
Repo schema: 8ee590a366600f34f1748ffdfa24d389c13f85fa

hh-fc9898fbdf5f3dca608e09fd83e663a644608bec-3.23.4 Thu Jan 11 17:46:28 2018

### Operating System and Version

CentOS Linux release 7.3.1611

### Standalone code, or other way to reproduce the problem

```
// summon a mysterious storm
$rval = $memcached->cas($casKey, $key, $value, $expires);
var_dump($rval, $memcached->getResultCode() === Memcached::RES_SUCCESS);
```

### Actual result
```
bool(false)
bool(true)
```

### Expected result
```
bool(false)
bool(false)
```

Some (small) percentage of the time, memcached updates (`cas`, `delete`, `set`) return `false`.
A percentage of *those* times, `getResultCode` provides `Memcached::RES_SUCCESS`, even though the return value was false.

About 82% of the times where an update returns `false` but `getResultCode` provides `Memcached::RES_SUCCESS`, the value is *indeed* updated with the correct value.
The other 18% of the time where an update returns `false` but `getResultCode` provides `Memcached::RES_SUCCESS`, the value is *not* updated with the correct value, but it's possible that it was manipulated in another thread.

I can't reproduce this case manually, but we see it hundreds of thousands of times a day, with
non-weird keys/values.

Does anyone have any insight here?

My problem is mostly that the return value doesn't match `getResultCode` - I want to know that if the return value is false, then the value didn't get written to memcached.

Thanks for any help