Make roll_time a `std::time::Duration` #54

Job	Run time
build	1m 45s
fmt	7s
clippy	1m 36s
tests (mobile-packet-verifier)	1m 20s
tests (ingest)	1m 28s
tests (file-store)	1m 20s
tests (reward-scheduler)	31s
tests (task-manager)	35s
tests-postgres (boost-manager)	2m 29s
tests-postgres (iot-config)	2m 28s
tests-postgres (iot-packet-verifier)	3m 23s
tests-postgres (iot-verifier)	2m 13s
tests-postgres (mobile-config)	1m 49s
tests-postgres (mobile-verifier)	2m 6s
build-release	0s
	23m 10s

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