Add biggest cell related info into web ui

There are some disadvantages to large cell, such as can't be cached or cause memory fragmentation, but currently user can't easily to find them out.

My proposal is save len and key of the biggest cell into fileinfo of hfile, and shown on web ui, including two places.

1: Add "Len Of Biggest Cell" into main page of regionServer, in here we can find out which regions has large cell by sorting.

2: Add "Len Of Biggest Cell" and "Key Of Biggest Cell" into region page, in here we can find out the exactly key and the hfile.