Get rid of fill_free call from the create_pool

[pfreixes]

The idea is allow further pools to do not give support to the fill_free internal method, putting it hide at the current implementation and making it a none hard requirement.

Further pools can decide to do it in another ways, leaving future connections openned perhaps.

To keep backward compatibility with the current one the fill free might be called using an ensure future wrapped with an event to signal future waiters.

Thoughts?