Prevent mongo connector to count more than 1M rows

[davinov]

Change Summary

Trying to get_slice on a very large collection (25M rows) was very slow (1min30).
This is caused by the count facet, which forces mongo to scan all docs regardless of the limit.

I suggest we limit this to a somewhat high value, but that doesn't impact too much performance.

I choose 1M, because it took 3s on my local mongo, and seems a nice threshold above which the exact count is not very relevant.

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable

[codecov-io]

Codecov Report

Merging #159 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #159   +/-   ##
=======================================
  Coverage   97.52%   97.52%           
=======================================
  Files          42       42           
  Lines        2097     2098    +1     
=======================================
+ Hits         2045     2046    +1     
  Misses         52       52           

Impacted Files	Coverage Δ
toucan_connectors/mongo/mongo_connector.py	100.00% <100.00%> (ø)

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[testinnplayin]

C'est super !