ObjectPool for pages and a SlicingArena

[JohannesLichtenberger]

Page instances should be recycled, when the BufferManager evicts pages,as we currently always allocate new instances. Instead we should preallocate "empty" instances and clear them once evicted from the cache and put the instances back into one or two ObjectPools (for IndirectPages and KeyValueLeafPages). In the off-heap branch we should also allocate a big memory chunk upfront and use a slicing allocator to slice into smaller chunks for the KeyValueLeafPages.