cleanup: use typedef for private message ID's in callback

[JFreegman]

This change is 

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.09%. Comparing base (7b1db6a) to head (32e67ab).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2729 +/- ## ========================================== - Coverage 73.13% 73.09% -0.04% ========================================== Files 149 149 Lines 30516 30516 ========================================== - Hits 22317 22305 -12 - Misses 8199 8211 +12 ```

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