A call to  nc_utils.wait_next_block_factory(eth)() crashes with  [Errno 24] Too many open files

1) Start hydrachain -d datadir runmultiple --num_validators=3 --seed=42
2) Open a console Ctrl+C, Enter
3) Execute calls

```
from hydrachain.nc_utils import wait_next_block_factory
nc_utils.wait_next_block_factory(eth)() crashes with
```

The call crashes with the error: [Errno 24] Too many open files
