[sqllab] log query fetch time

[timifasubaa]

This PR adds logging for the time spent fetching results from the db cursor

@john-bodley

[codecov-io]

Codecov Report

Merging #6033 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6033      +/-   ##
==========================================
+ Coverage   64.63%   64.67%   +0.04%     
==========================================
  Files         449      449              
  Lines       23821    23823       +2     
  Branches     2639     2639              
==========================================
+ Hits        15396    15408      +12     
+ Misses       8412     8402      -10     
  Partials       13       13

Impacted Files	Coverage Δ
superset/sql_lab.py	71.76% <100%> (+0.33%)	⬆️
superset/connectors/sqla/models.py	81.13% <0%> (+0.75%)	⬆️
superset/db_engine_specs.py	55.7% <0%> (+0.87%)	⬆️

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