fix(postgres): remove if not exists and cascade on create/drop db

[ALiangLiang]

Pull Request check-list

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Description of change

Resolved #10030

Postgresql doesn't support CREATE DATABASE IF NOT EXISTS.... So remove words IF NOT EXISTS.

Add more options on postgresql CREATE DATABASE:

• template
• ctype

refs:

• https://www.postgresql.org/docs/9.5/static/sql-createdatabase.html
• https://www.postgresql.org/docs/9.5/static/sql-dropdatabase.html

[codecov]

Codecov Report

Merging #10033 into master will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##           master   #10033   +/-   ##
=======================================
  Coverage    96.3%    96.3%           
=======================================
  Files          63       63           
  Lines        9421     9421           
=======================================
  Hits         9073     9073           
  Misses        348      348

Impacted Files	Coverage Δ
lib/dialects/mysql/query-generator.js	97.84% <ø> (ø)	⬆️
lib/query-interface.js	89.8% <0%> (ø)	⬆️
lib/dialects/postgres/query-generator.js	93.98% <100%> (ø)	⬆️

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[sushantdhiman]

Thanks @ALiangLiang