Refactor: libcrmcommon: Clarify fractional seconds in time format

The use of "%N" and "nano_*" were misleading. We only support storing
and displaying times at microsecond resolution or less.

• Storing: Note that we fetch hr_dt->useconds and append three zeros unconditionally in the string representation (as nanoseconds).
• Displaying: Note the nano_digits = QB_MIN(nano_digits, 6). This caps the displayed field width so that the fractional (decimal) component of a timestamp is at microsecond resolution or coarser. So we can never display any of the three zeros that were appended above.

We have to keep "%N" and the existing behavior for backward
compatibility, of course.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>