Support Rails 6.1

[AnD00]

I'd like to add Rails 6.1 support for activerecord-turntable.

In this PR, the turntable itself is judged whether the version of activerecord is 6.1 or later, and the process is branched and executed.

Since 6.1, activerecord has been changed drastically to support multiple DBs, etc., it may be simpler to make turntable itself a major version up and not support 6.0 or lower.

Please let me know if there is anything else I need to do.

[gussan]

TODO:

• Upgrade rspec-rails to 4.x or later ( rspec-rails supports Rails 6.1)
• Support Ruby 3.0.x (Rails 6.1.x supports Ruby >= 2.5.0 and <= 3.0.x)
  • Fix compatibility with Ruby 3.0.x keyword arguments
• [want] Support ar-import

Issue about Ruby 3.0.x

Ruby 3.0 changes about keyword arguments may break some turntable methods that have options argument.

For example:

  32) ActiveRecord::Turntable::ActiveRecordExt::Persistence When the model is sharded by surrogate key When updating
      Failure/Error:
        shard.connection.transaction(options) do
          yield
        end

      ArgumentError:
        wrong number of arguments (given 1, expected 0)
      # ./lib/active_record/turntable/cluster.rb:52:in `shards_transaction'
      # ./lib/active_record/turntable/connection_proxy.rb:22:in `shards_transaction'
      # ./lib/active_record/turntable/active_record_ext/transactions.rb:19:in `with_transaction_returning_status'
      # ./spec/factories/users.rb:7:in `block (3 levels) in <top (required)>'
      # ./spec/active_record/turntable/active_record_ext/persistence_spec.rb:23:in `block (3 levels) in <top (required)>'
      # ./spec/active_record/turntable/active_record_ext/persistence_spec.rb:31:in `block (4 levels) in <top (required)>'

This should be changed to receive keyword arguments.

Issue about ar-import

On rails 3.1.x, AR-import's SQL may be changed to no FROM clause.

  1) ActiveRecord::Turntable::ActiveRecordExt::ActiverecordImportExt With sequencer enabled model is expected not to raise Exception
     Failure/Error: it { is_expected.not_to raise_error }

       expected no Exception, got #<NoMethodError: undefined method `size' for nil:NilClass> with backtrace:
         # ./lib/active_record/turntable/mixer.rb:129:in `build_select_fader'
         # ./lib/active_record/turntable/mixer.rb:40:in `build_fader'
         # ./lib/active_record/turntable/connection_proxy.rb:93:in `method_missing'
         # ./spec/active_record/turntable/active_record_ext/activerecord_import_ext_spec.rb:6:in `block (4 levels) in <top (required)>'
         # ./spec/active_record/turntable/active_record_ext/activerecord_import_ext_spec.rb:15:in `block (3 levels) in <top (required)>'