Store DateTime as nanos in docstore

[trinity-1686a]

we use BinarySerializable to encode datetimes into the doc store. Strangely, while DateTime are precise up to ns, we only store µs there. We should store the value we got with full precision instead

tantivy/common/src/datetime.rs

Lines 166 to 176 in 99a59ad

	impl BinarySerializable for DateTime {
	fn serialize<W: Write + ?Sized>(&self, writer: &mut W) -> std::io::Result<()> {
	let timestamp_micros = self.into_timestamp_micros();
	<i64 as BinarySerializable>::serialize(&timestamp_micros, writer)
	}
	fn deserialize<R: Read>(reader: &mut R) -> std::io::Result<Self> {
	let timestamp_micros = <i64 as BinarySerializable>::deserialize(reader)?;
	Ok(Self::from_timestamp_micros(timestamp_micros))
	}
	}

[fulmicoton]

We need to be careful with backward compatiblity with that one.

Is it motivated by a problem observed on quickwit side? Is there a ticket open somewhere?

[trinity-1686a]

it was observed that when asked to return nanoseconds, quickwit returns values truncated to microsecond precision. This is surprising, and potentially problematic for use-cases relying on high precision timestamps