Avoid making extra 5 arrays in each `save`

Each `save` calls `all_timestamp_attributes_in_model` to fill timestamp
columns. Allthough the `all_timestamp_attributes_in_model` returns the
same value every time, the `all_timestamp_attributes_in_model` makes
extra 5 arrays every time.

This avoids the making extra 5 arrays by memoizing the result, it makes
`save` economical and a bit faster.

https://gist.github.com/kamipo/1ddad2235073f508637bf9a72d64bb83

Before (2a015f6c0be0593a624b0c800e5335319ac4c660):

```
{["~/rails/activerecord/lib/active_record/timestamp.rb",
  76,
  :T_ARRAY]=>[1000, 0, 341, 0, 1, 13640],
 ["~/rails/activerecord/lib/active_record/timestamp.rb",
  64,
  :T_ARRAY]=>[1000, 0, 341, 0, 1, 13640],
 ["~/rails/activerecord/lib/active_record/timestamp.rb",
  80,
  :T_ARRAY]=>[1000, 0, 341, 0, 1, 13640],
 ["~/rails/activerecord/lib/active_record/timestamp.rb",
  68,
  :T_ARRAY]=>[1000, 0, 341, 0, 1, 13640],
 ["~/rails/activerecord/lib/active_record/timestamp.rb",
  73,
  :T_ARRAY]=>[1000, 0, 341, 0, 1, 13640]}
Warming up --------------------------------------
    User.create * 10    36.000  i/100ms
Calculating -------------------------------------
    User.create * 10    353.644  (± 7.4%) i/s -      1.764k in   5.021876s
```

After (this change):

```
{["~/rails/activerecord/lib/active_record/timestamp.rb",
  83,
  :T_ARRAY]=>[1, 0, 1, 1, 1, 40],
 ["~/rails/activerecord/lib/active_record/timestamp.rb",
  87,
  :T_ARRAY]=>[1, 0, 1, 1, 1, 40],
 ["~/rails/activerecord/lib/active_record/timestamp.rb",
  64,
  :T_ARRAY]=>[1, 1, 1, 1, 1, 0],
 ["~/rails/activerecord/lib/active_record/timestamp.rb",
  69,
  :T_ARRAY]=>[1, 1, 1, 1, 1, 0],
 ["~/rails/activerecord/lib/active_record/timestamp.rb",
  74,
  :T_ARRAY]=>[1, 1, 1, 1, 1, 0]}
Warming up --------------------------------------
    User.create * 10    37.000  i/100ms
Calculating -------------------------------------
    User.create * 10    380.063  (± 7.1%) i/s -      1.924k in   5.097917s
```

0 files changed, 0 insertions, 0 deletions