Fix client event buffering

[Shatur]

I rushed a little with removing generics.
We need to keep separate buffer for each event, otherwise events of
different types encoded in Bytes will be mixed together.

This commit is partial revert of e98a940.

[UkoeHB]

Add test for multiple event types?

[codecov]

Codecov Report

Attention: Patch coverage is 87.30159% with 8 lines in your changes missing coverage. Please review.

Project coverage is 89.77%. Comparing base (52a92fa) to head (9d8be99).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/core/event_registry/server_event.rs	69.23%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
- Coverage   89.79%   89.77%   -0.03%     
==========================================
  Files          37       37              
  Lines        2293     2317      +24     
==========================================
+ Hits         2059     2080      +21     
- Misses        234      237       +3     

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[Shatur]

@UkoeHB done!