dashdemux: support ISO8601 durations that overflow

ISO-8601 durations can contain hours, minutes and seconds, each
of which is optional. Each of these can be an unbounded number
and are not limited to 24 hours, 60 minutes or 60 seconds.

gstmpdparser was making the assumption that there was a limit on
the size of hours, minutes and seconds, which means that it is
unable to parse a DASH manifest that makes use of this feature of
ISO8601.

Fixes #840
4 jobs for issue840-iso8601-duration-overflow in 5 minutes and 38 seconds (queued for 1 second)
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