De-duplicate subscription calls

[ajatprabha]

When using Option such as OnConnectHandler, the delegated paho library calls it in a separate goroutine as soon as the connection is open to a broker.

This change makes sure that in case of a shared subscription, actual subscription is called only once per client.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cd5858c) 99.70% compared to head (c93c894) 99.72%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   99.70%   99.72%   +0.01%     
==========================================
  Files          32       32              
  Lines         681      720      +39     
==========================================
+ Hits          679      718      +39     
  Misses          2        2              

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