[9.4] Make full_page_writes only settable on server start?

There is a significant amount of code supporting the changing of
full_page_writes while the server is running, including an
XLOG_FPW_CHANGE wal record. But I don't see the use-case; surely almost
nobody changes this on a running server, because either your filesystem
guarantees atomic page writes for you or not.

I'd like to submit a patch to just make it into a PGC_POSTMASTER and
remove the code to support changing it.

Then, I intend to write another patch to make the full-page writes for
checksums honor the full_page_writes setting. That will be easier to
write once it's a PGC_POSTMASTER.

Any reason to keep the setting as a PGC_SIGHUP?

Regards,Jeff Davis