Close connection on DISCONNECT message

[adam-fowler]

Close connection after receiving DISCONNECT message from server
also fix reading of last buffer

[codecov-commenter]

Codecov Report

Merging #87 (33bffca) into main (ef331a6) will decrease coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #87      +/-   ##
==========================================
- Coverage   77.41%   77.15%   -0.27%     
==========================================
  Files          21       19       -2     
  Lines        2183     2092      -91     
==========================================
- Hits         1690     1614      -76     
+ Misses        493      478      -15     

Impacted Files	Coverage Δ
...s/MQTTNIO/ChannelHandlers/MQTTMessageDecoder.swift	89.89% <100.00%> (+4.33%)	⬆️
Sources/MQTTNIO/MQTTListeners.swift	73.68% <0.00%> (-26.32%)	⬇️
Sources/MQTTNIO/MQTTClientV5.swift	62.92% <0.00%> (-15.74%)	⬇️
Sources/MQTTNIO/MQTTProperties.swift	76.47% <0.00%> (-1.69%)	⬇️
Sources/MQTTNIO/MQTTConnection.swift	91.07% <0.00%> (-1.20%)	⬇️
Sources/MQTTNIO/MQTTClient.swift	77.35% <0.00%> (-0.76%)	⬇️
...MQTTNIO/AsyncAwaitSupport/MQTTClientV5+async.swift
...s/MQTTNIO/AsyncAwaitSupport/MQTTClient+async.swift
Sources/MQTTNIO/MQTTPacket.swift	84.75% <0.00%> (+3.36%)	⬆️
... and 1 more

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