Batching page logging during B-tree build

Hi!

There's a thread about GiST build [0]. And we've found out there that logging newly created index with batches of 32
pagesis slightly faster. Heikki implemented new logging routine log_newpages() for GiST build and it makes code for
thisbatching nice and clean. 
Here is PoC with porting that same routine to B-tree. It allows to build B-trees ~10% faster on my machine.

Thanks!

Best regards, Andrey Borodin.

[0]
https://www.postgresql.org/message-id/flat/12CC4B83-9A5D-4A59-85A1-215CF9D1AB4B%40yandex-team.ru#6b93ee95fcc51ba710715d2ff6017b7f