fix microseconds leading zeroes

[Baxulio]

We have TIMESTAMPTZ field with value: '2023-07-31 16:19:49.91874'
if we map it to trantor::Date class and invoke ::toDbLocalString() it returns '2023-07-31 16:19:49.091874'

The microseconds part is converted incorrectly, because 91874 is not equal to 091874. It should be '2023-07-31 16:19:49.918740'

e.g. https://stackoverflow.com/questions/62937930/postgres-timestamp-with-microseconds