refactor(cursor): simplify implicit cursor session creation

This code used to live in each of the cursor subclasses in order
to ensure sessions creation never happened in the core driver. Now
that we are merged, we can move this into the core cursor in order
to simply refactoring.

NODE-2065

lib/command_cursor.js

@@ -159,8 +159,7 @@ var methodsToInherit = [
   'explain',
   'isNotified',
   'isKilled',
-  '_endSession',
-  '_initImplicitSession'
+  '_endSession'
 ];
 
 // Only inherit the types we need

lib/cursor.js

@@ -216,21 +216,21 @@ if (SUPPORTS.ASYNC_ITERATOR) {
 
 // Map core cursor _next method so we can apply mapping
 Cursor.prototype._next = function() {
-  if (this._initImplicitSession) {
-    this._initImplicitSession();
-  }
   return CoreCursor.prototype.next.apply(this, arguments);
 };
 
 for (let name in CoreCursor.prototype) {
   Cursor.prototype[name] = CoreCursor.prototype[name];
 }
 
-Cursor.prototype._initImplicitSession = function() {
+Cursor.prototype._initializeCursor = function(callback) {
+  // implicitly create a session if one has not been provided
   if (!this.s.explicitlyIgnoreSession && !this.s.session && this.s.topology.hasSessionSupport()) {
     this.s.session = this.s.topology.startSession({ owner: this });
     this.cursorState.session = this.s.session;
   }
+
+  CoreCursor.prototype._initializeCursor.apply(this, [callback]);
 };
 
 Cursor.prototype._endSession = function() {

lib/operations/explain.js

@@ -14,10 +14,6 @@ class ExplainOperation extends OperationBase {
 
   execute() {
     const cursor = this.cursor;
-
-    if (cursor._initImplicitSession) {
-      cursor._initImplicitSession();
-    }
     return CoreCursor.prototype.next.apply(cursor, arguments);
   }
 }