keepalive: trace in case of success

cpp-rust-driver tests sometimes depend upon logs emitted by the driver.
HeartbeatTests are on of them. Tests look for a following log:
`logger_.add_critera("Heartbeat completed on host " + ccm_->get_ip_prefix());`.

Before this commit, rust-driver was not emitting any logs in case of
keepalive request success. I think it's not a bad idea to add a trace log
here, considering that keepalive intervals are rather long (default is 30s).

Obviously, the logger criteria will need to
be adjusted in `cpp-rust-driver` tests as well.

Author: muzarski

scylla/src/transport/connection.rs

@@ -1839,6 +1839,11 @@ impl Connection {
                     );
                     return Err(err);
                 }
+
+                trace!(
+                    "Keepalive request successful on connection to node {}",
+                    node_address
+                );
             }
         } else {
             // No keepalives are to be sent.