Make couch_btree:chunkify/1 prefer fewer chunks
Make couch_btree:chunkify/1 prefer fewer chunks

This changes couch_btree:chunkify/1 to produce fewer larger chunks
rather than creating chunks of even-ish size.

COUCHDB-3298

fkaempfer/couchdb,fkaempfer/couchdb,apache/couchdb-couch,cloudant/couchdb-couch,cloudant/couchdb-couch,apache/couchdb-couch,fkaempfer/couchdb,apache/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,cloudant/couchdb-couch