`make-dir-store` is not atomic

[LiberalArtist]

While looking at a Discourse thread, I noticed that make-dir-store uses (with-output-to-file #:exists 'replace ...):

web-server/web-server-lib/web-server/stuffers/store.rkt

Lines 9 to 13 in 87a1e58

	(with-output-to-file
	(build-path home (bytes->string/utf-8 key))
	(lambda ()
	(write value))
	#:exists 'replace))

It seems like it would be better to use call-with-atomic-output-file here: if somehow you ended up with a partially-written file, it seems like it would be very painful to debug.