Simplify exchange rate storage logic in OrderProcessor #183

Job	Run time
CI / Context	4s
CI / Generate matrix	11s
CI / Check governance	4s
CI / 8.3 prf-low mariadb phpunit all	3m 7s
CI / 8.4 mysql84 phpunit all	3m 52s
CI / 8.3 mysql80 phpunit all	4m 8s
CI / 8.3 mysql80 phplinting	2m 14s
CI / Dispatch tag patch release	-1s
	13m 39s

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