max_pool_size -> maxPoolSize

PyMongo doesn't currently support the maxPoolSize URI option. Instead it provides a max_pool_size named parameter for MongoClient. This causes a few problems:

• The MongoDB URI isn't fully portable between PyMongo and other drivers.
• max_pool_size is treated as a PyMongo specific option

For PyMongo 3.0, the max_pool_size named parameter will be removed. Support for the maxPoolSize URI option will be added. maxPoolSize can be passed in a MongoDB URI, or as a keyword argument for MongoClient (similar to other URI options supported through **kwargs).