Defer plugin callbacks to next tick

Nock 13.x has changed to better reflect the semantics of modern Node versions.
A side effect of this was the test
`CloudantClient/#db using listeners/with ComplexPlugin2` failing with the `end` event being triggered twice, causing two done callbacks.
The changes suggest that nock moved the abort event to the next tick. This ensures our plugin callbacks also move so that abort is handled correctly.

Author: ricellis

lib/clientutils.js

@@ -1,4 +1,4 @@
-// Copyright © 2017, 2018 IBM Corp. All rights reserved.
+// Copyright © 2017, 2021 IBM Corp. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -67,7 +67,7 @@ var processState = function(r, callback) {
   if (r.abort) {
     // [1] => Client has called for the request to be aborted.
     abort();
-    callback(new Error('Client issued abort')); // no retry
+    setImmediate(callback, new Error('Client issued abort')); // no retry
     return;
   }
 
@@ -77,15 +77,15 @@ var processState = function(r, callback) {
     sendResponseToClient(r.state.abortWithResponse, r.clientStream, r.clientCallback);
     var err = new Error('Plugin issued abort');
     err.skipClientCallback = true;
-    callback(err); // no retry
+    setImmediate(callback, err); // no retry
     return;
   }
 
   if (r.state.retry && r.state.attempt < r.state.maxAttempt) {
     // [3] => One or more plugins have called for the request to be retried.
     abort();
     debug('Plugin issued a retry.');
-    callback(); // retry
+    setImmediate(callback); // retry
     return;
   }
 
@@ -104,7 +104,7 @@ var processState = function(r, callback) {
 
   if (!r.state.sending) {
     // [4] => Request has not yet been sent. Still processing 'onRequest' hooks.
-    callback(); // continue
+    setImmediate(callback); // continue
     return;
   }
 
@@ -120,7 +120,7 @@ var processState = function(r, callback) {
   }
 
   // [5] => Return response to awaiting client.
-  callback(new Error('No retry requested')); // no retry
+  setImmediate(callback, new Error('No retry requested')); // no retry
 };
 
 // execute a specified hook for all plugins