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Rewrite MongoStore initialization
* Re-use existing or upcoming mongoose connection
* Re-use existing or upcoming node-mongodb-native connection
* Accept full-featured MongoDB connection strings + advanced options
* Compatible with legacy config
* Replace callback by `connected` event
* Add debug

Fix #51, #58, #62, #66, #70, #85, #94, #96, #115, #117, #120
Fix #124, #128, #129, #130, #131, #133, #134