There are two places which try to release the next waiter, when the
ongoing connection is being canceled and we call explicitly to wake up
a waiter or when the connection is being released. In both situations we
call the same method `_release_waiter` that will iterate per all of the
queues till found one that has a waiter that is not already canceled.

This iteration will also check that the limit and the limit per host is
not overcomed. The logic to check how many available connections/free
slots there are has been placed behind the `_available_connections`
function.

So far this PR fixes:

- Take into account the limit and limit per host at any time that we try
to wake up a waiter. No availables, no wake up.
- Dont give up till reach a waiter that is not finished.
- Iterate always through all of the different hosts.

What is missing:

- More coverage
- Shuffle the keys to give always the same chances to all of the hosts?