Make roll_time a `std::time::Duration` (#820) #58

Job	Run time
fmt	5s
clippy	1m 39s
build	1m 51s
tests (file-store)	1m 45s
tests (mobile-packet-verifier)	1m 38s
tests (ingest)	1m 34s
tests (reward-scheduler)	33s
tests (task-manager)	41s
tests-postgres (boost-manager)	2m 26s
tests-postgres (iot-config)	2m 12s
tests-postgres (iot-packet-verifier)	3m 11s
tests-postgres (iot-verifier)	2m 7s
tests-postgres (mobile-config)	1m 54s
tests-postgres (mobile-verifier)	2m 7s
build-release	0s
	23m 43s

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