Added method to IM to remove objects by class and id. Then used it to remove objects when updating counters.

Clear IdentityMap before continue this test, we can do this here because store_full_sti_class is not supposed to change during "runtime".

This reverts commit 4db9dca55e3acc2c59f252eb83ecb83db5f4b81b.

Conflicts:

	activerecord/test/cases/identity_map_test.rb

Use association_class method which returns the reflection class, this method is redefined in polymorphic belongs to associations.

Check if constant is defined in AR, if not this can cause errors when using polymorphic associations.

Use strings primary keys in identity map keys to avoid problems with casting and also allow strings primary keys.

implicit_readonly is not set until records are loaded, just check readonly_value and then set readonly status.

Don't use identity map if loading readonly records, this will prevent changing readonly status on already loaded records.

Remove objects from identity map if save! failed, otherwise finding again the same record will have invalid attributes.