Summary:
`PinnableSlice` may hold a handle to a cache value which must be released to correctly decrement the ref-counter. However, when `PinnableSlice` variables are reused, e.g. like this:
```
PinnableSlice pin_slice;
db.Get("foo", &pin_slice);
db.Get("foo", &pin_slice);
```
then the second `Get` simply overwrites the old value in `pin_slice` and the handle returned by the first `Get` is _not_ released.

This PR adds `Reset` calls to the `Get`/`MultiGet` calls that accept `PinnableSlice` arguments to ensure proper cleanup of old values.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10166

Reviewed By: hx235

Differential Revision: D37151632

Pulled By: ajkr

fbshipit-source-id: 9dd3c3288300f560531b843f67db11aeb569a9ff