perf: mesgdef clear array pool

[muktihari]

• Clear fields' array pool after we are done using it. A field may be an unknown field and field's Value may contains pointer to a slice. In the case of unknown field, its FieldBase object is newly created, it's not referencing factory's defined object, so it needs to be freed. Previously, those objects can only be freed after the array pool is freed first since the array pool is still referencing them. Now those objects can be freed right after we clear the array pool.
• Minor clean code, update mesgdef code documentation for SetUnknownFields and SetDeveloperFields.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (dfc5d4e) to head (b7bb55d).

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #478   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           42        42           
  Lines         3810      3810           
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  Hits          3810      3810           

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