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.
Cleanup of DataKeyTop serialization.

Legacy encoding is the only encoding type that uses CodecBin2
for this specific data attachment.
Merge pull request #83 from addthis/keytopper-fix2-export

KeyTopper and page serialization improvements.