Add foreground eviction

[johnramsden]

Use a channel mechanism for either eviction on a interval, or immediately when triggered through a channel request.

When eviction is triggered it drains the channel and informs all that eviction occurred.

---

WIP, currently Deadlocks at:

OxCache/oxcache/src/cache/mod.rs

Lines 149 to 151 in b7cd945

	pub async fn remove_zones(&self, zone_indices: &[usize]) -> tokio::io::Result<()> {
	let mut map_guard = self.buckets.write().await; // DEADLOCK
	let mut reverse_mapping_guard = self.zone_to_entry.lock().await;

[johnramsden]

I'm not sure I like this but this is what we have for now. It kind of busy waits. we might need to adjust

OxCache/oxcache/src/device.rs

Lines 251 to 272 in b7cd945

	let zone_index = loop {
	match self.get_free_zone() {
	Ok(res) => break res,
	Err(err) => {
	eprintln!("[append] Failed to get free zone: {}", err);
	}
	};
	let (resp_tx, resp_rx) = flume::bounded(1);
	if let Err(e) = self.eviction_channel.send(EvictorMessage {
	sender: resp_tx,
	}) {
	eprintln!("[append] Failed to send eviction message: {}", e);
	return Err(std::io::Error::new(
	std::io::ErrorKind::Other,
	"Failed to send eviction message",
	));
	};
	if let Err(e) = resp_rx.recv() {
	eprintln!("[append] Failed to receive eviction message: {}", e);
	}
	};

[2a46m4]

It looks good to me!