Cache results of computing model type

We faced a significant performance decrease when we started using STI
without storing full namespaced class name in type column (because of PostgreSQL
length limit for ENUM types).
We realized that the cause of it is the slow STI model instantiation. Problematic
method appears to be `ActiveRecord::Base.compute_type`, which is used to find
the right class for STI model on every instantiation.
It builds an array of candidate types and then iterates through it calling
`safe_constantize` on every type until it finds appropriate constant. So if
desired type isn't the first element in this array there will be at least one
unsuccessful call to `safe_constantize`, which is very expensive, since it's
defined in terms of `begin; rescue; end`.

This commit is an attempt to speed up `compute_type` method simply by caching
results of previous calls.

```ruby
class MyCompany::MyApp::Business::Accounts::Base < ApplicationRecord
  self.table_name = 'accounts'
  self.store_full_sti_class = false
end

class MyCompany::MyApp::Business::Accounts::Free < Base
end

class MyCompany::MyApp::Business::Accounts::Standard < Base
  # patch .compute_type there
end

puts '======================= .compute_type ======================='
Benchmark.ips do |x|
  x.report("original method") do
    MyCompany::MyApp::Business::Accounts::Free.send :compute_type, 'Free'
  end
  x.report("with types cached") do
    MyCompany::MyApp::Business::Accounts::Standard.send :compute_type, 'Standard'
  end
  x.compare!
end
```

```
======================= .compute_type =======================
  with types cached:  1529019.4 i/s
    original method:     2850.2 i/s - 536.46x  slower
```

```ruby
5_000.times do |i|
  MyCompany::MyApp::Business::Accounts::Standard.create!(name: "standard_#{i}")
end

5_000.times do |i|
  MyCompany::MyApp::Business::Accounts::Free.create!(name: "free_#{i}")
end

puts '====================== .limit(100).to_a ======================='
Benchmark.ips do |x|
  x.report("without .compute_type patch") do
    MyCompany::MyApp::Business::Accounts::Free.limit(100).to_a
  end
  x.report("with .compute_type patch") do
    MyCompany::MyApp::Business::Accounts::Standard.limit(100).to_a
  end
  x.compare!
end
```

```
====================== .limit(100).to_a =======================
     with .compute_type patch:      360.5 i/s
  without .compute_type patch:       24.7 i/s - 14.59x  slower
```

1 files changed, 4 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index b59d24f88d..5069c6666f 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Optimize slow model instantiation when using STI and `store_full_sti_class = false` option.
+
+    *Konstantin Lazarev*
+
 *   Add `touch` option to counter cache modifying methods.
 
     Works when updating, resetting, incrementing and decrementing counters: