balanced destination shard as observer

[bogdan-rosianu]

different destination shard as an observer when no shard is specified. same as #2960 but targeted to master branch

Testing procedure:

• start an observer by using this branch
• if the key is not in consensus and the node will start as observer, it should be routed to a "random" shard. The same key in multiple tries should be routed to the same shard.
• try with multiple keys in order to see the shard balancing

[codecov]

Codecov Report

Merging #2992 (77fc4f0) into master (3d326fa) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 77fc4f0 differs from pull request most recent head 2a3cc95. Consider uploading reports for the commit 2a3cc95 to get more accurate results

@@           Coverage Diff           @@
##           master    #2992   +/-   ##
=======================================
  Coverage   75.02%   75.02%           
=======================================
  Files         620      620           
  Lines       59086    59094    +8     
=======================================
+ Hits        44329    44337    +8     
  Misses      10803    10803           
  Partials     3954     3954           

Impacted Files	Coverage Δ
core/converters.go	86.90% <100.00%> (+1.37%)	⬆️

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[gabi-vuls]

Ssytem test passed