Add auto-reconnect option

[dvbmgr]

Hi,

Please find my first pull request to this project. It aims to enable the option to automatically try to reconnect to the LDAP server if the connection is closed. As I already mentionned in an issue, I am quite new to Elixir, so feel free to adapt this PR to what suits best.

In particular, if the new configuration retry_on_connection_closed is set to true, then each call to GenServer.call is observed by do_call_or_retry/2. If it returns a {:error, :ldap_closed} or a {:error, {:gen_tcp_error, term}} , then it calls Paddle.reconnect and resends the request.

I have not been able to find an equivent of :gen_tcp_error for the :ssl-module, but I think, based on eldap.erl's do_send/3, that both (maybe erroneously) return :gen_tcp_error.

I sadly wasn't able to reproduce the configuration for the tests, but I don't expect any to fail. But that would be something to check for in the QA.

It is tricky to write tests for this PR, and I wasn't really able to do so. My approach to ensure a correct behaviour:

• for :ldap_closed errors, that is, when :eldap is able to close the connection properly, I just killed the socket using ss. It would be quite easy to implement, but I wasn't comfortable with adding this as a dependency, as it would be quite hard to setup for the users anyway.
• for :gen_tcp_error, it's quite tricky, and I just played with my LDAP server a little bit. What should really be done in order to properly insure that it behaves as expected would be using a tool that allows for connections drop, as for instance toxiproxy does. But that's a lot of work, that I'm not willing to put in right now, and again, there is the extra dependency issue.

With my best regards !

[dvbmgr]

Actually, I should probably update the specs of each function that calls do_call_and_retry too. I'll fix that in the next few days.