Make Monero and Bitcoin wallet use a generalized sync interval

We define the sync interval as 1/10th of the blocktime. For the
special case of our tests, we however check at max once per second.
The tests have a super fast blocktime. As such we shouldn't hammer
the nodes with a request every 100ms.

swap/src/env.rs

@@ -1,4 +1,5 @@
 use crate::bitcoin::{CancelTimelock, PunishTimelock};
+use std::cmp::max;
 use std::time::Duration;
 use time::NumericalStdDurationShort;
 
@@ -15,6 +16,16 @@ pub struct Config {
     pub monero_network: monero::Network,
 }
 
+impl Config {
+    pub fn bitcoin_sync_interval(&self) -> Duration {
+        sync_interval(self.bitcoin_avg_block_time)
+    }
+
+    pub fn monero_sync_interval(&self) -> Duration {
+        sync_interval(self.monero_avg_block_time)
+    }
+}
+
 pub trait GetConfig {
     fn get_config() -> Config;
 }
@@ -75,3 +86,26 @@ impl GetConfig for Regtest {
         }
     }
 }
+
+fn sync_interval(avg_block_time: Duration) -> Duration {
+    max(avg_block_time / 10, Duration::from_secs(1))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn check_interval_is_one_second_if_avg_blocktime_is_one_second() {
+        let interval = sync_interval(Duration::from_secs(1));
+
+        assert_eq!(interval, Duration::from_secs(1))
+    }
+
+    #[test]
+    fn check_interval_is_tenth_of_avg_blocktime() {
+        let interval = sync_interval(Duration::from_secs(100));
+
+        assert_eq!(interval, Duration::from_secs(10))
+    }
+}