Use H5_ATOMIC macros in H5VL #56

Job	Run time
build-and-test (--enable-api-tests, --enable-multithread --disable-hl, production)	4m 53s
build-and-test (--enable-api-tests, --enable-multithread --disable-hl, debug)	4m 37s
build-and-test (--enable-api-tests, --disable-multithread --enable-threadsafe --disable-hl, produ...	7m 32s
build-and-test (--enable-api-tests, --disable-multithread --enable-threadsafe --disable-hl, debug)	9m 9s
build-and-test (--enable-api-tests, --disable-multithread --disable-threadsafe, production)	12m 34s
build-and-test (--enable-api-tests, --disable-multithread --disable-threadsafe, debug)	15m 8s
build-and-test (--disable-api-tests, --enable-multithread --disable-hl, production)	4m 4s
build-and-test (--disable-api-tests, --enable-multithread --disable-hl, debug)	3m 33s
build-and-test (--disable-api-tests, --disable-multithread --enable-threadsafe --disable-hl, prod...	7m 48s
build-and-test (--disable-api-tests, --disable-multithread --enable-threadsafe --disable-hl, debug)	9m 19s
build-and-test (--disable-api-tests, --disable-multithread --disable-threadsafe, production)	12m 56s
build-and-test (--disable-api-tests, --disable-multithread --disable-threadsafe, debug)	14m 25s
	1h 45m 58s

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