Fails to read big dir in parallel

Preparation

```
mkdir many
for i in {1..100000} ; do touch many/$i ; done
```

Failed

```
require 'eventmachine'
require 'eio/eventmachine'

EM.run do
  EIO.eventmachine_handler
  EIO.readdir('many') {|f|  }
  EIO.readdir('many') {|f|  }
  EM.add_timer(5){ EM.stop }
end
```
