Add write API endpoint to write data unsync'd

[pauldix]

We should have a new write endpoint (or perhaps a parameter on the existing one?) that will accept writes and put it into the WAL buffer and return a success to the client prior to the WAL flush happening. It should still validate, it just won't wait for the WAL file to be persisted to object storage to return.

[hiltontj]

To add a bit to this, I think a lot of the "plumbing" is set up to do this already.

Right now, when we handle an incoming write, we use the write_lp method on the write buffer, which validates the write into a WalOp::Write (and a WalOp::Catalog if necessary), then send it to the WAL with the Wal::write_ops method:

influxdb/influxdb3_write/src/write_buffer/mod.rs

Lines 251 to 264 in b7fd8e2

	// if there were catalog updates, ensure they get persisted to the wal, so they're
	// replayed on restart
	let mut ops = Vec::with_capacity(2);
	if let Some(catalog_batch) = result.catalog_updates {
	ops.push(WalOp::Catalog(catalog_batch));
	}
	ops.push(WalOp::Write(result.valid_data));
	// write to the wal. Behind the scenes the ops get buffered in memory and once a second (or
	// whatever the configured wal flush interval is set to) the buffer is flushed and all the
	// data is persisted into a single wal file in the configured object store. Then the
	// contents are sent to the configured notifier, which in this case is the queryable buffer.
	// Thus, after this returns, the data is both durable and queryable.
	self.wal.write_ops(ops).await?;

That is what is waiting for WAL durability before responding to the caller.

I think the key to this is to use the Wal trait's buffer_op_unconfirmed method:

influxdb/influxdb3_wal/src/lib.rs

Lines 63 to 64 in b7fd8e2

	/// Buffer into a single larger operation in memory. Returns before the operation is persisted.
	async fn buffer_op_unconfirmed(&self, op: WalOp) -> Result<(), Error>;

That could be called first with the WalOp::Catalogs, if there are any, then the WalOp::Writes to accomplish what is described above before returning a response.

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FWIW, I think a parameter on the existing write API would work well.