feat(ReplicaSet): set electionTimeoutMillis to 500ms instead of 10s as default value

* electionTimeoutMillis default to 500ms

* All defaults in constructor

* Changed default to avoid accidental overriding

* Changed default to avoid accidental overriding

Author: fracasula

README.md

@@ -158,24 +158,6 @@ const replSet = new MongoMemoryReplSet({
 });
 ```
 
-The `replSet.configSettings` can be used to tune the initialization of the replica set. By default one of the nodes has
-to be promoted to Primary before the replica set can be successfully used but that happens only after a timeout of 10 seconds
-(see [electionTimeoutMillis](https://docs.mongodb.com/manual/reference/replica-configuration/#rsconf.settings.electionTimeoutMillis)).
-
-It is possible to force the replica set to elect a node as Primary before the default 10 seconds by changing the `electionTimeoutMillis`
-parameter as shown below:
-
-```js
-const replSet = new MongoMemoryReplSet({
-  replSet: {
-    configSettings: {
-      electionTimeoutMillis: 100,
-    },
-  },
-});
-await replSet.waitUntilRunning();
-```
-
 ### Simple test with MongoClient
 
 Take a look at this [test file](https://github.com/nodkz/mongodb-memory-server/blob/master/src/__tests__/singleDB-test.ts).

src/MongoMemoryReplSet.ts

@@ -224,7 +224,10 @@ export default class MongoMemoryReplSet extends EventEmitter {
       const rsConfig = {
         _id: this.opts.replSet.name,
         members,
-        settings: this.opts.replSet.configSettings || {},
+        settings: {
+          electionTimeoutMillis: 500,
+          ...(this.opts.replSet.configSettings || {}),
+        },
       };
       await this.admin.command({ replSetInitiate: rsConfig });
       this.debug('Waiting for replica set to have a PRIMARY member.');