Reasonable backoff strategies

[mempirate]

Context

Our current backoff strategy is just retrying every 100ms. This should be configurable, and we should support both linear and exponential backoff strategies.

The current code is here:

msg-rs/msg-transport/src/durable/session.rs

Lines 155 to 197 in 8f99004

	fn on_disconnect(mut self: Pin<&mut Self>, cx: &mut Context<'_>) {
	// We copy here because we can't do it after borrowing self in the match below
	let endpoint = self.endpoint;
	match &mut self.state {
	SessionState::Connected(_) => {
	error!("Session was disconnected from {}", self.endpoint);
	self.state = SessionState::Disconnected(ReconnectStatus {
	attempts: 0,
	current_attempt: Some(Io::establish(self.endpoint)),
	});
	}
	SessionState::Disconnected(reconnect_status) => {
	debug!(
	attempts = reconnect_status.attempts,
	"Reconnect failed, retrying..."
	);
	reconnect_status.attempts += 1;
	// Start and set the new reconnect attempt
	let attempt = Box::pin(async move {
	tokio::time::sleep(Duration::from_millis(100)).await;
	Io::establish(endpoint).await
	});
	reconnect_status.current_attempt = Some(attempt);
	}
	SessionState::Processing(_) => {
	error!("Session was disconnected from {}", self.endpoint);
	self.state = SessionState::Disconnected(ReconnectStatus {
	attempts: 0,
	current_attempt: Some(Io::establish(self.endpoint)),
	});
	}
	SessionState::Terminated(_) => {
	unreachable!("Session was already terminated")
	}
	}
	// Register the waker to make sure the reconnect attempt is polled
	cx.waker().wake_by_ref();
	}