311 adapter lifecycle foundations

[bretambrose]

Misc. functionality needed to support connect/disconnect and lifecycle events in the 311_to_5 adapter

• Adds support for internal observers to reset the mqtt5 client's connection when it is established or being established.
• Reworks how ref-counting and state changes are protected and adds a simple helper function to safely make callbacks based on synchronized state
• Splits ref-counting into external and internal. Internal is all ref-counts made inside the implementation in order to extend the lifetime of the adapter long enough to safely handle asynchronous operations. External is ref counts made by users. The adapter now follows a contract where as soon as the last external ref count is released, no more callbacks to user code will be made under any circumstances. This greatly simplifies shutdown from a user perspective (it definitely complicates things internally).
• Move the adapter's locking approach to a read-write lock rather than a mutex.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Patch coverage: 53.78% and project coverage change: -0.18 ⚠️

Comparison is base (d3377e4) 80.34% compared to head (96d45cb) 80.17%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #293      +/-   ##
==========================================
- Coverage   80.34%   80.17%   -0.18%     
==========================================
  Files          18       18              
  Lines        7571     7625      +54     
==========================================
+ Hits         6083     6113      +30     
- Misses       1488     1512      +24     

Impacted Files	Coverage Δ
source/mqtt.c	94.59% <ø> (ø)
source/v5/mqtt5_client.c	87.63% <8.33%> (-0.72%)	⬇️
source/mqtt3_to_mqtt5_adapter.c	45.27% <58.87%> (+2.77%)	⬆️

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