Extend ttlval to accept ISO 8601 durations
The ttlval configuration types are replaced by duration configuration types. The duration is an ISO 8601 duration that is going to be used for DNSSEC key timings such as key lifetimes, signature resign intervals and refresh periods, etc. But it is also still allowed to use the BIND ttlval ways of configuring intervals (number plus optional unit). A duration is stored as an array of 7 different time parts. A duration can either be expressed in weeks, or in a combination of the other datetime indicators. Add several unit tests to ensure the correct value is parsed given different string values.
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