Fix email verification token index

The original index, "emails.validationTokens.token" is never used in any meteor packages.  A search of "validationTokens" across all the packages in `meteor/packages` returns nothing.

The correct index should be, "services.email.verificationTokens.token".  This is used in the `verifyEmail` method to locate the correct user record.  Without a matching index, this causes a full table scan each time `verifyEmail` is called.

Fixes #4482.

History.md

@@ -100,6 +100,10 @@
   `AccountsServer` constructors, so that users can create multiple
   independent instances of the `Accounts` namespace.  #4233
 
+* Create an index for `Meteor.users` on
+  `services.email.verificationTokens.token` (instead of
+  `emails.validationTokens.token`, which never was used for anything).  #4482
+
 ### Minimongo
 
 * The `$push` query modifier now supports a `$position` argument.  #4312

packages/accounts-password/password_server.js

@@ -779,7 +779,7 @@ Accounts.createUser = function (options, callback) {
 ///
 /// PASSWORD-SPECIFIC INDEXES ON USERS
 ///
-Meteor.users._ensureIndex('emails.validationTokens.token',
+Meteor.users._ensureIndex('services.email.verificationTokens.token',
                           {unique: 1, sparse: 1});
 Meteor.users._ensureIndex('services.password.reset.token',
                           {unique: 1, sparse: 1});