Rework the notion of Epoch #2720

Job	Run time
build_and_test (openssl)	7m 9s
build_and_test (rustcrypto, alloc)	7m 13s
build_and_test (mbedtls)	7m 24s
build_and_test (rustcrypto, os)	26m 23s
build_and_test (mbedtls, alloc)	7m 39s
build_and_test (mbedtls, os)	11m 22s
build_and_test (rustcrypto)	9m 45s
build_and_test (openssl, os)	9m 40s
build_and_test (openssl, alloc)	7m 32s
	1h 34m 7s

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