Deprecate "primary" as a connection_specification_name for ActiveRecord::Base

[eileencodes]

As multiple databases have evolved it's becoming more and more
confusing that we have a connection_specification_name that defaults
to "primary" and a spec_name on the database objects that defaults to
"primary" (my bad).

Even more confusing is that we use the class name for all
non-ActiveRecord::Base abstract classes that establish connections. For
example connections established on class MyOtherDatabaseModel < ApplicationRecord will use "MyOtherDatabaseModel" as it's connection
specification name while ActiveRecord::Base uses "primary".

This PR deprecates the use of the name "primary" as the
connection_specification_name for ActiveRecord::Base in favor of
using "ActiveRecord::Base".

In this PR the following is true:

• If handler.establish_connection(:primary) is called, "primary"
  will not throw a deprecation warning and can still be used for the
  connection_specification_name. This also fixes a bug where using this
  method to establish a connection could accidentally overwrite the actual
  ActiveRecord::Base connection IF that connection was not using a
  configuration named :primary.
• Calling handler.retrieve_connection "primary" when
  handler.establish_connection :primary has never been called will
  return the connection for ActiveRecord::Base and throw a deprecation
  warning.
• Calling handler.remove_connection "primary" when
  handler.establish_connection :primary has never been called will
  remove the connection for ActiveRecord::Base and throw a deprecation
  warning.

See #38179 for details on more motivations for this change.

Co-authored-by: John Crepezzi john.crepezzi@gmail.com