[ENH] Server embedder: use queue, handle unsuccessful requests at the end

[PrimozGodec]

Issue

Server embedder retries sending unsuccessful requests immediately after the fauliture.
Since it is possible that the server needs longer to compute (the request was unsuccessful because of timeout) the results (the server will cache the result when finished) it is better to retry later (there is a higher chance that the server already finished processing).

Description of changes

Implemented described behaviour. To achieve it I needed to change embedders to use Queue instead of just gathering the results.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #5835 (f3a9dde) into master (ba151c8) will increase coverage by 0.01%.
The diff coverage is 98.38%.

@@            Coverage Diff             @@
##           master    #5835      +/-   ##
==========================================
+ Coverage   86.24%   86.25%   +0.01%     
==========================================
  Files         315      315              
  Lines       66672    66695      +23     
==========================================
+ Hits        57503    57531      +28     
+ Misses       9169     9164       -5     

[markotoplak]

@PrimozGodec, I like the concept and structuring. I left you some minor comments and suggestions.

In the end, I think I found:

• 2 3 imaginary serious bugs, where I was only complaining because I did not understand the code properly.
• 1 bug that I still believe in. :)

[PrimozGodec]

@markotoplak it should be ready now