Hooking into `execute` and `exec_query` is problematic
because some adapters like MySQL2 have `exec_query` call `execute`
which forces QueryLogs to first check wether the comment was already
applied.

Using a prepended module is also a bit problematic because it
means it has to be prepended to the "final" adapter classes
but if the user application has a custom adapter that inherits
from a built-in one, the built-in one no longer have QueryLogs
working.

So instead this PR introduce `ActiveRecord.query_transformers`,
and the adpters are responsible for applying to transformers only
once.