Check inherited socket

Hello,

This patchset is about adding an extra redis option, `check_inherited_socket`, that allows to control the fd leakage on fork (`Redis::InheritedError`)

There are cases when you want the child to inherit the redis socket to avoid an unnecessary reconnect. In that case you need to handle locking yourself.

```
redis.ping # connect to redis
thousands_of_times do
  fork do
    # share redis socket and avoid reconnecting
    redis.set 'foo', 'bar'
  end
end
```

The `check_inherited_socket` name is probably a bad one, we can change it with something more appropriate.