Use symbols instead of strings

ActiveSupport::Concern is used all over Rails
This PR will only create 3 new objects as keys are never recreated and
are not subject to garbage collection.

The strings were being uniquely created and garbage collected. I don't
have any performance numbers but this should be better than all of the
GC.
Making the rake file for tests easier to read.

I'm defining a new class which modularizes how the `rake test` tasks are
defined and invoked.
Improved grammar and replaced 'dbs' slang with 'databases'
Removed support for deprecated `delete_sql` in associations.
Merge pull request #11254 from neerajdotname/delete-sql-be-gone

Removed support for deprecated `delete_sql` in associations.
Dropped deprecated option `:restrict` for `:dependent` in associations
Merge pull request #11255 from neerajdotname/restrict-be-gone

Dropped deprecated option `:restrict` for `:dependent` in associations
deprecated `ActiveRecord::TestCase` is no longer public.

/cc @tenderlove
remove unused `ActiveRecord::TestCase#sqlite3` method.
Adding actionview in Rakefile to run test 

and create versions
remove wrong documentation from a :nodoc:'d method
Merge pull request #11257 from arunagw/adding_actionview

Adding actionview in Rakefile to run test [ci skip]
eagerly initialize the attributes module to avoid check-then-set race conditions
initialize ivars so we don't have to constantly check them
Remove deprecated SchemaStatements#distinct
we don't need to use active support *everwhere* common.
define attribute methods in a thread safe manner
remove private attribute reader
lock around mutating the generated methods module
fix to_sql output on eager loaded relations