libtracker-common: Do not miss subsecond info printing iso8601 dates
We used %T as a shortcut to print times, but that translates to %H:%M:%S and misses subsecond information. To avoid that, check whether the passed GDateTime contains subsecond information and resort to %H:%M:%S.%f in that case.
Since we prefer shorter iso8601 strings for storage purposes, combine this handling with the existing paths for timezone information, so iso8601 strings that don't need either can cut down those extra chars.
Fixes subsecond information being lost on database inserts, and possibly other date comparison/printing misbehaviors.