Re: Vectored I/O in bulk_write.c

Here's a rebase. I decided against committing this for v17 in the
end. There's not much wrong with it AFAIK, except perhaps an
unprincipled chopping up of writes with large io_combine_limit due to
simplistic flow control, and I liked the idea of having a decent user
of smgrwritev() in the tree, and it probably makes CREATE INDEX a bit
faster, but... I'd like to try something more ambitious that
streamifies this and also the "main" writeback paths. I shared some
patches for that that are counterparts to this, over at[1].

[1] https://www.postgresql.org/message-id/flat/CA%2BhUKGK1in4FiWtisXZ%2BJo-cNSbWjmBcPww3w3DBM%2BwhJTABXA%40mail.gmail.com