Allocation on demand in transactions

[kamipo]

Currently 1,000 transactions creates 10,000 objects regardless whether
it is necessary or not.

This makes allocation on demand in transactions, now 1,000 transactions
creates required 5,000 objects only by default.

ObjectSpace::AllocationTracer.setup(%i{path line type})

pp ObjectSpace::AllocationTracer.trace {
  1_000.times { User.create }
}.select { |k, _| k[0].end_with?("transaction.rb") }

Before (95d038f):

{["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  209,
  :T_HASH]=>[1000, 0, 715, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  210,
  :T_OBJECT]=>[1000, 0, 715, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  210,
  :T_HASH]=>[1000, 0, 715, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  80,
  :T_OBJECT]=>[1000, 0, 715, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  8,
  :T_ARRAY]=>[1000, 0, 715, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  81,
  :T_ARRAY]=>[1000, 0, 715, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  289,
  :T_STRING]=>[1000, 0, 714, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  116,
  :T_ARRAY]=>[1000, 0, 714, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  120,
  :T_ARRAY]=>[1000, 0, 714, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  121,
  :T_HASH]=>[1000, 0, 714, 0, 1, 0]}

After (this change):

{["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  213,
  :T_HASH]=>[1000, 0, 739, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  214,
  :T_OBJECT]=>[1000, 0, 739, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  214,
  :T_HASH]=>[1000, 0, 739, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  81,
  :T_OBJECT]=>[1000, 0, 739, 0, 1, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/abstract/transaction.rb",
  304,
  :T_STRING]=>[1000, 0, 738, 0, 1, 0]}