Migrate MongoDB client from unmaintained 3rd party library to official one

[philippgille]

When gokv was started, the 3rd-party library github.com/globalsign/mgo was the most popular and mature MongoDB client library for Go. But that changed and the official library go.mongodb.org/mongo-driver is now the proper go-to library. mgo is not maintained anymore and seems to not be compatible with newer versions of the MongoDB server anymore.

This PR updates the MongoDB library from github.com/globalsign/mgo to go.mongodb.org/mongo-driver.

I haven't tested the new gokv implementation with an older MongoDB server version (where mgo still worked), but from the way of how the document is defined, inserted/updated, read and deleted it could be compatible.

I'll add a breaking change notice to the CHANGELOG as a warning in any case. (separately)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0e6beda) 60.56% compared to head (6f1def7) 63.64%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   60.56%   63.64%   +3.08%     
==========================================
  Files          25       25              
  Lines        2097     2107      +10     
==========================================
+ Hits         1270     1341      +71     
+ Misses        716      651      -65     
- Partials      111      115       +4     

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