Rework the notion of Epoch #2725

Job	Run time
build_and_test (mbedtls)	7m 46s
build_and_test (rustcrypto, os)	26m 28s
build_and_test (openssl, alloc)	6m 57s
build_and_test (rustcrypto)	9m 51s
build_and_test (rustcrypto, alloc)	5m 44s
build_and_test (mbedtls, alloc)	6m 27s
build_and_test (openssl, os)	9m 54s
build_and_test (mbedtls, os)	10m 35s
build_and_test (openssl)	7m 4s
	1h 30m 46s

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