Active socket, implementation with state machine (#68)

* Switch to gen_statem for MLLP.Client

* Added Unit test for detection of disconnected receiver

* Replaced some mock tests with real ones

* Make sure socket is invalidated on reconnection

* Fix flaky test (disconnect on server side could result either in :closed or :einval depending on timing of send call)

* Add 'responses match requests' test for concurrent requests

* Fix TestDispatcher (ask receiver to slow down, if we want it to)

* Handle 'data after trailer' case

* Switch backoff tests from mock to real

* Add 'send_timeout_close' to socket defaults

* Shutdown on unexpected packet

* Shutdown socket on receive timeout, if required

* Update cache version

---------

Co-authored-by: Bryan Paxton <39971740+starbelly@users.noreply.github.com>

.github/workflows/main.yml

@@ -11,7 +11,7 @@ env:
   MIX_ENV: test
   otp-version: '24'
   elixir-version: '1.14'
-  cache-version: '2'
+  cache-version: '3'
 
 jobs:
   build: