perf(serializer): Avoid unnecessary serialization of detached objects

[JR-Morgan]

Previously, we were calling ujson.dumps for each object, for each transport.
Under normal circumstances, there are 2 transports (server + sqlite), which means everything was being serialized twice, which is not necessary.

This is in the hotpath; worth adressing.

Easy optimisation, with small but non-trivial performance improvement.
I don't understand cprofile's flame graph. It seems the size of the box doesn't properly correlate with the execution time.
But I'm seeing 33s -> 21s for ujson.dumps calls

Before:

After:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.01%. Comparing base (3bcdf72) to head (1bd764d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #431   +/-   ##
=======================================
  Coverage   90.01%   90.01%           
=======================================
  Files         130      130           
  Lines        6099     6100    +1     
=======================================
+ Hits         5490     5491    +1     
  Misses        609      609           

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