Use correct db in a multi db setup

[glujan]

SQLCompiler has a .connection instance which is already aware of what
database it should use. As such, when running EXPLAIN query let's use
that connection instead of connection to a default database.

Fixes #522.

I will add unit tests early next week.

[codecov]

Codecov Report

Merging #538 (70d78cc) into master (2a9880e) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #538      +/-   ##
==========================================
- Coverage   84.60%   84.59%   -0.01%     
==========================================
  Files          51       51              
  Lines        2059     2058       -1     
==========================================
- Hits         1742     1741       -1     
  Misses        317      317              

Impacted Files	Coverage Δ
silk/sql.py	78.12% <100.00%> (-0.34%)	⬇️

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[glujan]

@auvipy I think tests should pass now and the PR is ready for review.

[auvipy]

let's see what CI says

[glujan]

Look like coverage has slightly dropped because from django.db import connection line was removed from silk/sql.py

[auvipy]

I want to see some more feedback