KeyTopper and page serialization improvements. 

1. Deprecation of the ConcurrentKeyTopper. Our data attachments are protected by a reader/writer lock so the ConcurrentKeyTopper adds runtime and serialization overheads that are unnecessary.
2. Rewrite of the KeyTopper class. Faithfully implements the top-K algorithm.
3. Convert all the KeyToppper data attachment to use the BytesCodable interface. This is necessary for converting the old KeyTopper implementation to the new KeyTopper implementation.
4. Use an unsigned variable length long as the header for each page object.