grpc-js: Unref backoff timer in subchannel

Author: murgatroid99

packages/grpc-js/src/subchannel.ts

@@ -120,6 +120,7 @@ export class Subchannel {
     this.backoffTimeout = new BackoffTimeout(() => {
       this.handleBackoffTimer();
     }, backoffOptions);
+    this.backoffTimeout.unref();
     this.subchannelAddressString = subchannelAddressToString(subchannelAddress);
 
     this.keepaliveTime = options['grpc.keepalive_time_ms'] ?? -1;