Rework the notion of Epoch #2718

Job	Run time
build_and_test (openssl, os)	9m 3s
build_and_test (openssl)	8m 8s
build_and_test (mbedtls)	7m 46s
build_and_test (openssl, alloc)	5m 55s
build_and_test (mbedtls, os)	9m 17s
build_and_test (mbedtls, alloc)	9m 3s
build_and_test (rustcrypto)	9m 17s
build_and_test (rustcrypto, os)	9m 17s
build_and_test (rustcrypto, alloc)	7m 10s
build_and_test (openssl, os)	10m 10s
build_and_test (mbedtls, os)	10m 43s
build_and_test (mbedtls, alloc)	8m 15s
build_and_test (rustcrypto)	9m 40s
build_and_test (rustcrypto, os)	26m 4s
build_and_test (mbedtls)	7m 46s
build_and_test (openssl, alloc)	5m 55s
build_and_test (rustcrypto, alloc)	7m 10s
build_and_test (openssl)	8m 8s
	2h 48m 47s

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