Call self.class only once in init_internals

Calling `self.class` multiple times is not cheap.

```ruby
class A
  def self.foo
  end

  def foo1
    self.class.foo
    self.class.foo
    self.class.foo
    self.class.foo
  end

  def foo2
    klass = self.class
    klass.foo
    klass.foo
    klass.foo
    klass.foo
  end
end

a = A.new

Benchmark.ips do |x|
  x.report("foo1") { a.foo1 }
  x.report("foo2") { a.foo2 }
end
```

```
Warming up --------------------------------------
                foo1   341.701k i/100ms
                foo2   414.000k i/100ms
Calculating -------------------------------------
                foo1      3.194M (± 5.4%) i/s -     16.060M in   5.044653s
                foo2      4.276M (± 3.8%) i/s -     21.528M in   5.041999s
```

Similar with rails#36052.

Author: kamipo

activerecord/lib/active_record/core.rb

@@ -837,17 +837,20 @@ def to_ary
       end
 
       def init_internals
-        @primary_key              = self.class.primary_key
         @readonly                 = false
         @previously_new_record    = false
         @destroyed                = false
         @marked_for_destruction   = false
         @destroyed_by_association = nil
         @_start_transaction_state = nil
-        @strict_loading           = self.class.strict_loading_by_default
-        @strict_loading_mode = self.class.strict_loading_mode
 
-        self.class.define_attribute_methods
+        klass = self.class
+
+        @primary_key         = klass.primary_key
+        @strict_loading      = klass.strict_loading_by_default
+        @strict_loading_mode = klass.strict_loading_mode
+
+        klass.define_attribute_methods
       end
 
       def initialize_internals_callback