Add invalidate! directly to Transaction

Every instance of `Transaction` including instances of `NullTransaction`
now respond to `invalidate!` which allows us to use transactions in
a "tell do not ask" way. This should simplify transaction invalidation
for cases were we needed check whether transaction is not a
`NullTransaction` to prevent calling `invalidate!` on `nil` state.

Author: nvasilevski

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

@@ -83,12 +83,15 @@ def add_record(record, _ = true); end
       def restartable?; false; end
       def dirty?; false; end
       def dirty!; end
+      def invalidate!; end
     end
 
     class Transaction # :nodoc:
       attr_reader :connection, :state, :savepoint_name, :isolation_level
       attr_accessor :written, :written_indirectly
 
+      delegate :invalidate!, to: :@state
+
       def initialize(connection, isolation: nil, joinable: true, run_commit_callbacks: false)
         @connection = connection
         @state = TransactionState.new
@@ -496,7 +499,7 @@ def within_new_transaction(isolation: nil, joinable: true)
                 begin
                   commit_transaction
                 rescue ActiveRecord::ConnectionFailed
-                  transaction.state.invalidate! unless transaction.state.completed?
+                  transaction.invalidate! unless transaction.state.completed?
                   raise
                 rescue Exception
                   rollback_transaction(transaction) unless transaction.state.completed?

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -1012,7 +1012,7 @@ def invalidate_transaction(exception)
           return unless exception.is_a?(TransactionRollbackError)
           return unless savepoint_errors_invalidate_transactions?
 
-          current_transaction.state.invalidate! if current_transaction.open?
+          current_transaction.invalidate!
         end
 
         def retryable_query_error?(exception)