TIMESTAMP_NS nanoseconds are truncated

E.g.
```(java)
java.sql.Timestamp ts = new java.sql.Timestamp(millis);
ts.setNanos(123456789);
ps.setTimestamp(1, ts); // column 1 has type TIMESTAMP_NS
```
DuckDBTimestamp converts the Timestamp to micros and as result nanosecond part (`789` from the example) is not stored.

The same happens on reading the value back