Make AR#increment! AR#decrement! concurency safe

`increment!` and `decrement!` should be concurency safe because
incorrect update can cause serious problems.

Especially when rails already have a concurency-safe counters update
mechanics in `.update_counters` method.

So this patch includes the following changes:

* Reuse code between increment and decrement methods
* Extracted a code snippet (search for ultimate 'eww' comment in diff)
  from has_many association internals to #increment! method
* Make use of `#increment!` and `#decrement!` methods where possible
  instead of `.update_counters` that doesn't update an counters on
  record instance level