refactor: support a factory type in connect

This allows for easier testing, as well as potentially supporting
multiple `Connection` types during our CMAP work

Author: mbroadst

lib/core/connection/connect.js

@@ -15,6 +15,7 @@ const MIN_SUPPORTED_SERVER_VERSION = WIRE_CONSTANTS.MIN_SUPPORTED_SERVER_VERSION
 let AUTH_PROVIDERS;
 
 function connect(options, callback) {
+  const ConnectionType = options && options.connectionType ? options.connectionType : Connection;
   if (AUTH_PROVIDERS == null) {
     AUTH_PROVIDERS = defaultAuthProviders(options.bson);
   }
@@ -26,7 +27,7 @@ function connect(options, callback) {
         return;
       }
 
-      performInitialHandshake(new Connection(socket, options), options, callback);
+      performInitialHandshake(new ConnectionType(socket, options), options, callback);
     });
 
     return;
@@ -40,13 +41,13 @@ function connect(options, callback) {
           return;
         }
 
-        performInitialHandshake(new Connection(ipv4Socket, options), options, callback);
+        performInitialHandshake(new ConnectionType(ipv4Socket, options), options, callback);
       });
 
       return;
     }
 
-    performInitialHandshake(new Connection(ipv6Socket, options), options, callback);
+    performInitialHandshake(new ConnectionType(ipv6Socket, options), options, callback);
   });
 }