Code review comment for lp:~kdub/mir/fix-1362444

So I audited the code by compiling by using
class BufferID : public IntWrapper<BufferIDTag, uint32_t>
{
BufferID(uint32_t val) : IntWrapper(val) {}
BufferID() = delete;
}

to catch anywhere that we were using the default constructor. It only being used in the RecentlyUsedCache and in some places in the tests. I improved these to not rely on the invalid concept.

I didn't see a good reason to change BufferID further, so kept the current use:
typedef IntWrapper<BufferIDTag, uint32_t> BufferID;