fix(relayer): Lower log severity on listener SIGKILL (#1780)

This is normal behaviour but it's interesting to know if the parent
does not register that the child exits, so continue to log on SIGKILL
but at a lower log severity.

src/clients/SpokePoolClient.ts

@@ -131,7 +131,7 @@ export class IndexedSpokePoolClient extends clients.SpokePoolClient {
       return;
     }
 
-    this.logger.warn({
+    this.logger[signal === "SIGKILL" ? "debug" : "warn"]({
       at: "SpokePoolClient#childExit",
       message: `${this.chain} SpokePool listener exited.`,
       code,