Remove correlated_subquery eager_limit_strategy
Remove deprecated model class dataset methods
Remove Model#pk_or_nil
Remove Sequel::Model.cache_anonymous_models accessor
Remove Model::InstaceMethods.class_attr_{overridable,reader}
Remove Model::EMPTY_INSTANCE_VARIABLES
Remove :one_to_one option check and eager_loader/grapher arity checks
No longer define class methods for public plugin dataset methods
Remove validates_not_string validation
Remove deprecated features of json_serializer and xml_serializer

This removes the :all_columns and :all_associations options from
both plugins.

In the json_serializer plugin, it makes :root=>true mean :root=>
:collection and not :root=>:both.  It removes Model.json_create,
and the adding and deleting of the JSON.create_id (:naked=>true
always).
Remove remaining use of the identity_map plugin (caught by Travis)
Make plugins extend ClassMethods before including InstanceMethods
Remove auto_validate_presence_columns from auto_validations plugin
Make validates_type not allow nil values without :allow_nil=>true
Assume database implements identifier quoting/mangling methods
Refactor adapter tests

Don't provide default database connections for adapter or
integration tests, for user to specify it via an environment
variable or spec_config.rb.

Always use INTEGRATION_DB in adapter tests, instead of a
per-adapter specific constant.

Use a consistent format for per-adapter environment variable
names, and allow SEQUEL_INTEGRATION_URL to work for adapter
tests as well as integration tests.

Remove SEQUEL_MSSQL_SPEC_REQUIRE and SEQUEL_DB2_SPEC_REQUIRE
environment variable handling, since RUBYOPT works just as well.