Rework the notion of Epoch #2739

Job	Run time
build_and_test (mbedtls)	7m 51s
build_and_test (mbedtls, alloc)	7m 55s
build_and_test (mbedtls, os)	11m 16s
build_and_test (rustcrypto, os)	26m 1s
build_and_test (openssl, os)	9m 28s
build_and_test (rustcrypto, alloc)	7m 25s
build_and_test (openssl, alloc)	7m 34s
build_and_test (openssl)	6m 55s
build_and_test (rustcrypto)	8m 9s
	1h 32m 34s

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