test: fix a number of our most notorious flakey tests

 - remove most `core/operations.test.js` tests using cursors in a
   sharded environment. We already have coverage for these commands
   and the returned batch sizes from our internal `_next` method
   are inconsistent

 - massive overhaul of the change streams tests which ensure that
   all async operations are awaited before cleanup, and that the
   change stream and client are closed in the correct order

 - refactor of the `defer` plugin to ensure that deferred actions
   happen LIFO, and in order rather than all at the same time

 - ensure `Long` is always sent for `getMore` commands, leading to
   flakey CSFLE tests

 - introduced `EventCollector` and renamd existing class to
   `APMEventCollector`

lib/cmap/wire_protocol/get_more.js

@@ -59,8 +59,13 @@ function getMore(server, ns, cursorState, batchSize, options, callback) {
     return;
   }
 
+  const cursorId =
+    cursorState.cursorId instanceof Long
+      ? cursorState.cursorId
+      : Long.fromNumber(cursorState.cursorId);
+
   const getMoreCmd = {
-    getMore: cursorState.cursorId,
+    getMore: cursorId,
     collection: collectionNamespace(ns),
     batchSize: Math.abs(batchSize)
   };

test/examples/change_streams.js

@@ -127,6 +127,11 @@ describe('examples(change-stream):', function() {
       });
       looper.run();
 
+      let processChange;
+      const streamExampleFinished = new Promise(resolve => {
+        processChange = resolve;
+      });
+
       // Start Changestream Example 3
       const collection = db.collection('inventory');
       const changeStream = collection.watch();
@@ -138,7 +143,7 @@ describe('examples(change-stream):', function() {
 
         newChangeStream = collection.watch({ resumeAfter: resumeToken });
         newChangeStream.on('change', next => {
-          // process next document
+          processChange(next);
         });
       });
       // End Changestream Example 3
@@ -155,6 +160,8 @@ describe('examples(change-stream):', function() {
       // End Changestream Example 3 Alternative
 
       await newChangeStreamIterator.close();
+
+      await streamExampleFinished;
       await newChangeStream.close();
       await looper.stop();