Broken connections are not repaired

[freeatnet]

Steps to reproduce:

1. Launch APN sender in a loop where the connection pool is persisted despite exceptions (i.e., Sidekiq).
2. Start sending notifications.
3. Simulate network problems.

Expected behaviour: Broken connections are repaired or discarded, failed messages are possibly retried.
Observed behaviour: Broken connections are supplied for the next messages.

[freeatnet]

The issue can be reproduced locally:

1. Check out freeatnet@489eb97
2. Run sample_keygen.rb in contrib. This should produce priv.pem and apn_production.pem, used by APN and the sample server.
3. Start sample_server.rb.
4. Simultaneously, start sample_client.rb. NB: this may require running it with bundle exec.
5. Ensure that messages are being sent successfully.
6. Press Enter a few times in the console running sample_server.rb to close some connections.
7. Note the error messages in sample_client.rb output.