1. $options[collection] could be an object, 2. turnable incrementHitCounter, 3. ensureIndex() on write only

1. Allow passing of MongoCollection in $options[collection] instead of a collection name (this is useful to economize connections if you already have an opened one and allow to specify replication-related custom options for a collection).
2. Option "incrementHitCounter" now defaults to false to minimize master database load.
3. ensureIndex() is now called on first write only to avoid touching the master database.