add a returnBrokenResource method to ShardedJedisPool

this way we can throw away broken ShardedJedis objects (e.g. due to timeouts on one shard)
example code for usage:
```
    while (true) { 
            ShardedJedis jedis = pool.getResource(); 
            try { 
                result = jedis.get(someObj); 
                break; 
            } catch (JedisException e) { 
                if (++count == maxTries) 
                    throw new IOException(e); 
                pool.returnBrokenResource(jedis); 
                jedis = pool.getResource(); 
            } finally { 
                pool.returnResource(jedis);              
            } 
        } 
```