Improve account counters handling (#15913)

* Improve account counters handling

* Use ActiveRecord::Base::sanitize_sql to pass values instead of interpolating them

Keep using string interpolation for `key` as it is safe and using
“ActiveRecord::Base::sanitize_sql_hash_for_assignment” would require stitching
bits of SQL in a way that is not more easily checked for safety.

* Add migration hook to catch PostgreSQL versions earlier than 9.5

spec/models/concerns/account_counters_spec.rb

@@ -0,0 +1,60 @@
+require 'rails_helper'
+
+describe AccountCounters do
+  let!(:account) { Fabricate(:account) }
+
+  describe '#increment_count!' do
+    it 'increments the count' do
+      expect(account.followers_count).to eq 0
+      account.increment_count!(:followers_count)
+      expect(account.followers_count).to eq 1
+    end
+
+    it 'increments the count in multi-threaded an environment' do
+      increment_by   = 15
+      wait_for_start = true
+
+      threads = Array.new(increment_by) do
+        Thread.new do
+          true while wait_for_start
+          account.increment_count!(:statuses_count)
+        end
+      end
+
+      wait_for_start = false
+      threads.each(&:join)
+
+      expect(account.statuses_count).to eq increment_by
+    end
+  end
+
+  describe '#decrement_count!' do
+    it 'decrements the count' do
+      account.followers_count = 15
+      account.save!
+      expect(account.followers_count).to eq 15
+      account.decrement_count!(:followers_count)
+      expect(account.followers_count).to eq 14
+    end
+
+    it 'decrements the count in multi-threaded an environment' do
+      decrement_by   = 10
+      wait_for_start = true
+
+      account.statuses_count = 15
+      account.save!
+
+      threads = Array.new(decrement_by) do
+        Thread.new do
+          true while wait_for_start
+          account.decrement_count!(:statuses_count)
+        end
+      end
+
+      wait_for_start = false
+      threads.each(&:join)
+
+      expect(account.statuses_count).to eq 5
+    end
+  end
+end