HPCC-8892 - Error on key build, with excessively large key+payload

Due to the internal 32k key+payload limit (impossed by legacy
header definition) - detect and fire an error if key+payload
exceeds the maximum.

Previously, if the keys managed to squeeze into key build process
but exceeded the maximum, then the key header 'length' was
truncated, causing a confusing error that key was larger than
this truncated value.

Signed-off-by: Jake Smith <jake.smith@lexisnexis.com>