sstables: add missing virtual destructor
sstables: add missing virtual destructor

A base class with virtual functions should also have a virtual destructor,
so if someone deletes it by the base class pointer, the concrete class's
destructor will be called.

I thought this missing virtual destructor is to blame for a bug I was
hunting, but it's not - but it's still worth adding this missing definition.

The silly "default" definition of the move constructor is also necessary,
because when you define the destructor explicitly, the compiler no longer
defines any constructors implicitly for you.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@cloudius-systems.com>

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