Fix potential leak when reusing PinnableSlice instances.

[mpoeter]

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.

[ajkr]

LGTM. I ran into this API pitfall once before too...

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