Mass removal of tokens in Meteor 1.7

This is urgent. Just found that this piece of `raix:push-update` method:

```
// xxx: Hack
// Clean up mech making sure tokens are uniq - android sometimes generate
// new tokens resulting in duplicates
var removed = Push.appCollection.remove({
$and: [
{ _id: { $ne: doc._id } },
{ token: doc.token }, // Match token
{ appName: doc.appName }, // Match appName
{ token: { $exists: true } } // Make sure token exists
]
});
```

...is deleting all tokens from `_raix_push_app_tokens` collection some times!

Start at the raix:push-update method and inspect the shown Push.appCollection.remove query against the _raix_push_app_tokens collection. Reproduce the mass-deletion case, determine why the duplicate-token cleanup matches unrelated records, and add regression coverage showing that only intended duplicates are removed.

Written by the indexing model from the issue text.