No duplicate vote inserts with mongodb

[r-marques]

This pr adds a unique compound index on the votes collection with node_pubkey and vote.voting_for_block. This prevents duplicated votes for the same public key for the same block.

Created and fixed tests.
Removed some unnecessary tests.

[codecov-io]

Codecov Report

Merging #1258 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1258   +/-   ##
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  Coverage   98.08%   98.08%           
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  Files          51       51           
  Lines        2295     2295           
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  Hits         2251     2251           
  Misses         44       44

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[ssadler]

This turns out to be kind of critical. A duplicate vote could take a block from being decided to being undecidable. This can't be mitigated by the server, the server can only detect when the database has incorrectly allowed a duplicate vote and throw an error.

[ssadler]

LGTM