A MongoDB adaptor for r2d2 connection pool.
```rust extern crate r2d2; extern crate r2d2_mongodb;
use r2d2::Pool; use r2d2_mongodb::{ConnectionOptionsBuilder, MongodbConnectionManager};
fn main () { let manager = MongodbConnectionManager::new( ConnectionOptionsBuilder::new() .withhost("localhost", 27017) .withdb("mydb") .with_auth("root", "password") .build() );
// let pool = Pool::builder()
// .max_size(16)
// .build(manager)
// .unwrap();
// ...
} ```