Remove unneeded indirection in code path.

[szmyd]

No API changes.

This just removes some of the clutter left from merging nuraft_grpc with sds_messaging and prepares for easier optional FlatBuffers serialization code path (while retaining Protobuf).

I also improved the logging and fixed a couple race issues in the UTs.

[codecov-commenter]

Codecov Report

Attention: 59 lines in your changes are missing coverage. Please review.

Comparison is base (8066124) 77.07% compared to head (ebd94dd) 74.51%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #46      +/-   ##
==========================================
- Coverage   77.07%   74.51%   -2.57%     
==========================================
  Files          20       19       -1     
  Lines         820      769      -51     
  Branches       91       90       -1     
==========================================
- Hits          632      573      -59     
- Misses        132      137       +5     
- Partials       56       59       +3     

Files	Coverage Δ
src/lib/manager_impl.hpp	50.00% <ø> (ø)
src/lib/mesg_factory.cpp	90.00% <100.00%> (ø)
src/lib/grpc_factory.cpp	74.31% <0.00%> (ø)
src/lib/common_lib.hpp	0.00% <0.00%> (ø)
src/lib/manager_impl.cpp	72.78% <65.33%> (-7.13%)	⬇️

... and 1 file with indirect coverage changes

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