HPCC-8892 - Avoid key max length issues for large key+payloads.

HPCC-8892 - Avoid key max length issues for large key+payloads.
    
Introduce a new 4byte max length header member to supplant the
exist 2byte length. Previously, a key+payload length that
exceeded the 2byte length, caused a truncated value to make it's
way into the header and the key build process to spuriously
complain that rows were too large adding.
    
The new member will be used for new keys, the old 'length' value
will still be maintained for backward compatibility.

-- 

Add a missing WINREV
Set a compatibility flag, if key size exceeds old max, that will
be detected by 702 builds