adding mongodb storage backend

[f00actual]

adding mongodb storage backend

closes #79

[majst01]

Nice so far, please check the output of golangci-lint run -p bugs -p unused

[majst01]

Can you please add some useful entries to the Readme as well, especially the table with the supported mongodb versions and some "normalized" benchmark numbers. According to the ci run the benchmark shows a ~ 2x performance of the postgres version.

[f00actual]

What approach did you take for the op/sec normalization in the README.md? It appears the latest bench results ran on different hardware and they might need updated overall. What do you think?

[majst01]

What approach did you take for the op/sec normalization in the README.md? It appears the latest bench results ran on different hardware and they might need updated overall. What do you think?

Like your current layout, but the problem persist, these hardware specs are hard to achieve. And what is actually running in the ci pipeline is not known.

[codecov-commenter]

Codecov Report

Merging #80 (d337d10) into master (10374d8) will decrease coverage by 1.62%.
The diff coverage is 63.55%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
- Coverage   75.84%   74.22%   -1.63%     
==========================================
  Files           8        9       +1     
  Lines         828      935     +107     
==========================================
+ Hits          628      694      +66     
- Misses        109      131      +22     
- Partials       91      110      +19     

Impacted Files	Coverage Δ
mongodb.go	63.55% <63.55%> (ø)
memory.go	93.10% <0.00%> (-3.45%)	⬇️

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[majst01]

I think this is ready for merging, please mark it as ready for review.