API: Refine meaning of duration field in Tank

Parent: [EPIC] API: Update naming and meaning of "track... (aura#215 - closed)

The duration field currently holds nanoseconds. As a consequence its use is not obvious, since all other service use seconds as their base unit (compare international system of units).

Please also check the parent / Engine services and their current datatype for duration.

Proposal

We have these options:

1. Keep the name duration but store seconds as float, instead of nanoseconds
2. Rename duration to durationNanoSeconds

Alternatively there is a ISO8601 Duration Definition, which is probably over-engineering for our use.