Deprecate and replace `set_state` method

`set_state` was directly setting the transaction state instance
variable. It's better to set the state via specific methods (`rollback!`
and `commit!` respectively.

While undocumented and untested, it's possible someone is using
`set_state` in their app or gem so I've added a deprecation notice to
it.

No where in the app do we use `nullify!` but I wanted to keep existing
behavior while replacing the method with a better pattern.

activerecord/CHANGELOG.md

@@ -1,3 +1,11 @@
*   Deprecate `set_state` method in `TransactionState`

    Deprecated the `set_state` method in favor of setting the state via specific methods. If you need to mark the
    state of the transaction you can now use `rollback!`, `commit!` or `nullify!` instead of
    `set_state(:rolledback)`, `set_state(:committed)`, or `set_state(nil)`.

    *Eileen M. Uchitelle*, *Aaron Patterson*

*   Deprecate delegating to `arel` in `Relation`.

    *Ryuta Kamizono*

activerecord/lib/active_record/connection_adapters/abstract/transaction.rb

@@ -1,8 +1,6 @@
module ActiveRecord
  module ConnectionAdapters
    class TransactionState
      VALID_STATES = Set.new([:committed, :rolledback, nil])

      def initialize(state = nil)
        @state = state
      end
@@ -24,10 +22,33 @@ def completed?
      end

      def set_state(state)
        unless VALID_STATES.include?(state)
        ActiveSupport::Deprecation.warn(<<-MSG.squish)
          The set_state method is deprecated and will be removed in
          Rails 5.2. Please use rollback! or commit! to set transaction
          state directly.
        MSG
        case state
        when :rolledback
          rollback!
        when :committed
          commit!
        when nil
          nullify!
        else
          raise ArgumentError, "Invalid transaction state: #{state}"
        end
        @state = state
      end

      def rollback!
        @state = :rolledback
      end

      def commit!
        @state = :committed
      end

      def nullify!
        @state = nil
      end
    end

@@ -57,7 +78,7 @@ def add_record(record)
      end

      def rollback
        @state.set_state(:rolledback)
        @state.rollback!
      end

      def rollback_records
@@ -72,7 +93,7 @@ def rollback_records
      end

      def commit
        @state.set_state(:committed)
        @state.commit!
      end

      def before_commit_records

activerecord/test/cases/transactions_test.rb

@@ -725,6 +725,44 @@ def test_transactions_state_from_commit
    assert transaction.state.committed?
  end

  def test_set_state_method_is_deprecated
    connection = Topic.connection
    transaction = ActiveRecord::ConnectionAdapters::TransactionManager.new(connection).begin_transaction

    transaction.commit

    assert_deprecated do
      transaction.state.set_state(:rolledback)
    end
  end

  def test_mark_transaction_state_as_committed
    connection = Topic.connection
    transaction = ActiveRecord::ConnectionAdapters::TransactionManager.new(connection).begin_transaction

    transaction.rollback

    assert_equal :committed, transaction.state.commit!
  end

  def test_mark_transaction_state_as_rolledback
    connection = Topic.connection
    transaction = ActiveRecord::ConnectionAdapters::TransactionManager.new(connection).begin_transaction

    transaction.commit

    assert_equal :rolledback, transaction.state.rollback!
  end

  def test_mark_transaction_state_as_nil
    connection = Topic.connection
    transaction = ActiveRecord::ConnectionAdapters::TransactionManager.new(connection).begin_transaction

    transaction.commit

    assert_equal nil, transaction.state.nullify!
  end

  def test_transaction_rollback_with_primarykeyless_tables
    connection = ActiveRecord::Base.connection
    connection.create_table(:transaction_without_primary_keys, force: true, id: false) do |t|