Redis cluster support for the r2d2 connection pool.

Example

```rust,norun extern crate r2d2; extern crate r2d2redis_cluster;

use std::thread;

use r2d2::Pool; use r2d2rediscluster::{Commands, RedisClusterConnectionManager};

fn main() { let redisuri = vec!["redis://127.0.0.1:6379", "redis://127.0.0.1:6378", "redis://127.0.0.1:6377"]; let manager = RedisClusterConnectionManager::new(redisuri).unwrap(); let pool = Pool::builder() .build(manager) .unwrap();

let mut handles = Vec::new();

for _ in 0..10 {
    let pool = pool.clone();
    handles.push(thread::spawn(move || {
        let connection = pool.get().unwrap();
        let n: u64 = connection.incr("test", 1).unwrap();
    }));
}

for h in handles {
    h.join().unwrap();
}

let connection = pool.get().unwrap();
let res: u64 = connection.get("test").unwrap();

assert_eq!(res, 10);

} ```