Fix order of disconnection and connection events

[Annopaolo]

In rare corner cases (time difference < 10 ms), the on_register hook (reconnection) may be called before the related on_client_offline/gone hook (disconnection). This is due to the distributed nature of VerneMQ (see https://github.com/vernemq/vernemq/issues/1741), and results in an invalid device connection state in Astarte: a device may be publishing while being in a disconnected status. Introduce a check on the order of disconnection/reconnection events, and possibly reorder them if such a corner case happens. During reordering (at most 50ms), other kind of messages (if any) are retained in VMQ memory.

Fix https://github.com/astarte-platform/astarte/issues/668.

[codecov[bot]]

Codecov Report

Merging #64 (8633c8f) into master (496beb1) will increase coverage by 5.09%. The diff coverage is 100.00%.

:exclamation: Current head 8633c8f differs from pull request most recent head 9bdb074. Consider uploading reports for the commit 9bdb074 to get more accurate results

@@            Coverage Diff             @@
##           master      #64      +/-   ##
==========================================
+ Coverage   73.18%   78.28%   +5.09%     
==========================================
  Files           9       11       +2     
  Lines         179      221      +42     
==========================================
+ Hits          131      173      +42     
  Misses         48       48              

Impacted Files	Coverage Δ
lib/astarte_vmq_plugin/application.ex	100.00% <ø> (ø)
lib/astarte_vmq_plugin.ex	93.54% <100.00%> (+0.56%)	:arrow_up:
lib/astarte_vmq_plugin/connection/synchronizer.ex	100.00% <100.00%> (ø)
...connection/synchronizer/synchronizer_supervisor.ex	100.00% <100.00%> (ø)

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