When freeing a file page, write a MLOG_INIT_FREE_PAGE record.
This allows us to avoid page flush and instead punch holes later,
in the page flushing. To implement that, we may want to make
buf_page_t::file_page_was_freed available in non-debug builds.

Crash recovery can choose to ignore or apply the record.

In BtrBulk::finish() we must not write this record, because
redo logging is being disabled for the page.