protocols/kad: Fix right shift overflow panic in record_received (#1492)

* protocols/kad: Add test to reproduce right shift overflow panic

* protocols/kad: Fix right shift overflow panic in record_received

Within `Behaviour::record_received` the exponentially decreasing
expiration based on the distance to the target for a record is
calculated as following:

1. Calculate the amount of nodes between us and the record key beyond
the k replication constant as `n`.

2. Shift the configured record time-to-live `n` times to the right to
calculate an exponentially decreasing expiration.

The configured record time-to-live is a u64. If `n` is larger or equal
to 64 the right shift will lead to an overflow which panics in debug
mode.

This patch uses a checked right shift instead, defaulting to 0 (`now +
0`) for the expiration on overflow.

* protocols/kad: Put attribute below comment

* protocols/kad: Extract shifting logic and rework test

Extract right shift into isolated function and replace complex
regression test with small isolated one.

* protocols/kad/src/behaviour: Refactor exp_decr_expiration

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>

protocols/kad/src/behaviour/test.rs

@@ -682,3 +682,15 @@ fn exceed_jobs_max_queries() {
         })
     )
 }
+
+#[test]
+fn exp_decr_expiration_overflow() {
+    fn prop_no_panic(ttl: Duration, factor: u32) {
+        exp_decrease(ttl, factor);
+    }
+
+    // Right shifting a u64 by >63 results in a panic.
+    prop_no_panic(KademliaConfig::default().record_ttl.unwrap(), 64);
+
+    quickcheck(prop_no_panic as fn(_, _))
+}