Make "default" an alias instead of the canonical driver name.

[SpacemanPaul]

Reason for this pull request

• The canonical name of the postgres driver was "default", with "postgres" and "legacy" as aliases.
• If dynamic driver loading failed, only the "postgres" driver was being forcibly injected.

Proposed changes

• The canonical name of the postgres driver is "postgres", with "default" and "legacy" as aliases.
• If dynamic driver loading fails, the "postgres", "postgis" and "memory" drivers are forcibly injected.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.44%. Comparing base (a5622d8) to head (51695e7).
Report is 36 commits behind head on develop-1.9.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop-1.9    #1590   +/-   ##
============================================
  Coverage        85.44%   85.44%           
============================================
  Files              140      140           
  Lines            15477    15479    +2     
============================================
+ Hits             13224    13226    +2     
  Misses            2253     2253           

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