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.
