Follow up to #22642.

[Joshua Cody & Yves Senn]

Closes #16757.

Prior to this patch schema loading rake tasks had the potential to leak a
connection to a different database. This had side-effects when rake tasks
operating on the current connection (like `db:seed`) were chained.

This extracts the logic that was embedded in a Rake task into a static
method.
Bonus: the first test for `rake db:migrate`

Closes #13625

When running tasks such "rake db:setup", instead of showing messages
like "db_development already exists", it was showing a big stack trace
and a message "Couldn't create database for ..." with the configuration
options, a very confusing message with a big trace.

This brings back the functionality present in 3-2, showing the same
message.

This reverts commit 90c887fa.

In a similar vein to Pat's work on create, drop etc, the db:charset
task is now a one liner in databases.rake