MB-28781 - scorch zap merge freq/loc copying only when fieldsSame
MB-28781 - scorch zap merge freq/loc copying only when fieldsSame

The optimization recently introduced in commit 530a3d24cf0768f4c7a,
("scorch zap optimize merge by byte copying freq/norm/loc's") was to
byte-copy freq/norm/loc data directly during merging.  But, it was
incorrect if the fields were different across segments.

This change now performs that byte-copying merging optimization only
when the fields are the same across segments, and if not, leverages
the old approach of deserializing & re-serializing the freq/norm/loc
information, which has the important step of remapping fieldID's.

See also: https://issues.couchbase.com/browse/MB-28781

blevesearch/bleve,blevesearch/bleve,steveyen/bleve,steveyen/bleve