macOS is a development platform for our product but calls to
fcntl(F_FULLFSYNC) are too slow for developer needs.  macOS offers
at least three different disk flush calls, making tradeoffs between
reliability and performance (see analysis notes on this MDEV).
On macOS only and as a compromise, we will back out the change to
call fcntl(F_FULLFSYNC), sacrificing a full flush in favor of more
performant writes.