Refactor RFQ BuyOrder struct #3328

Job	Run time
cross compilation	5m 18s
Docker build check	3m 32s
Compilation check	48s
Format check	22s
run itests	9m 0s
run itests postgres	16m 59s
Lint check	3m 5s
migration version check	10s
go mod check	22s
RPC check	1m 21s
run LiT tests	11m 40s
Sqlc check	26s
sample configuration check	47s
test vector check	55s
run unit tests (unit-race)	7m 22s
run unit tests (unit-cover)	2m 49s
run unit tests (unit dbbackend=postgres)	5m 23s
finish	3s
	1h 10m 22s

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