PERF: 20% faster pk attribute access

I've realized that `user.id` is 20% slower than `user.name` in the
benchmark (https://github.com/rails/rails/pull/35987#issuecomment-483882480).

The reason that performance difference is that `self.class.primary_key`
method call is a bit slow.

Avoiding that method call will make almost attribute access faster and
`user.id` will be completely the same performance with `user.name`.

Before (02b5b8cb):

```
Warming up --------------------------------------
             user.id   140.535k i/100ms
          user['id']    96.549k i/100ms
           user.name   158.110k i/100ms
        user['name']    94.507k i/100ms
       user.changed?    19.003k i/100ms
 user.saved_changes?    25.404k i/100ms
Calculating -------------------------------------
             user.id      2.231M (± 0.9%) i/s -     11.243M in   5.040066s
          user['id']      1.310M (± 1.3%) i/s -      6.565M in   5.012607s
           user.name      2.683M (± 1.2%) i/s -     13.439M in   5.009392s
        user['name']      1.322M (± 0.9%) i/s -      6.615M in   5.003239s
       user.changed?    201.999k (±10.9%) i/s -      1.007M in   5.091195s
 user.saved_changes?    258.214k (±17.1%) i/s -      1.245M in   5.007421s
```

After (this change):

```
Warming up --------------------------------------
             user.id   158.364k i/100ms
          user['id']   106.412k i/100ms
           user.name   158.644k i/100ms
        user['name']   107.518k i/100ms
       user.changed?    19.082k i/100ms
 user.saved_changes?    24.886k i/100ms
Calculating -------------------------------------
             user.id      2.768M (± 1.1%) i/s -     13.936M in   5.034957s
          user['id']      1.507M (± 2.1%) i/s -      7.555M in   5.017211s
           user.name      2.727M (± 1.5%) i/s -     13.643M in   5.004766s
        user['name']      1.521M (± 1.3%) i/s -      7.634M in   5.018321s
       user.changed?    200.865k (±11.1%) i/s -    992.264k in   5.044868s
 user.saved_changes?    269.652k (±10.5%) i/s -      1.344M in   5.077972s
```

1 files changed, 3 insertions, 3 deletions

diff --git a/activerecord/lib/active_record/core.rb b/activerecord/lib/active_record/core.rb
index b9046fcc1a..068ebf3c09 100644
--- a/activerecord/lib/active_record/core.rb
+++ b/activerecord/lib/active_record/core.rb
@@ -174,8 +174,7 @@ module ActiveRecord
 
         record = statement.execute([id], connection)&.first
         unless record
-          raise RecordNotFound.new("Couldn't find #{name} with '#{primary_key}'=#{id}",
-                                   name, primary_key, id)
+          raise RecordNotFound.new("Couldn't find #{name} with '#{key}'=#{id}", name, key, id)
         end
         record
       end
@@ -398,7 +397,7 @@ module ActiveRecord
     ##
     def initialize_dup(other) # :nodoc:
       @attributes = @attributes.deep_dup
-      @attributes.reset(self.class.primary_key)
+      @attributes.reset(@primary_key)
 
       _run_initialize_callbacks
 
@@ -570,6 +569,7 @@ module ActiveRecord
       end
 
       def init_internals
+        @primary_key              = self.class.primary_key
         @readonly                 = false
         @destroyed                = false
         @marked_for_destruction   = false