`object.transform_values!` returns `object` itself.

`Association#target=` invokes `loaded!`, so we no longer need to call
the `loaded!` explicitly.

Since Preloader is private API, we don't guarantee that it behaves like
older version as long as using Preloader directly. But this refactoring
fortunately also fix the Preloader compatibility issue #35195.

Closes #35195.

Fix elapsed time calculations

Fix `relation.exists?` with giving both `distinct` and `offset`

Bugfix has_many association #size when ids reader is cached and assoc…

I've found a few places in Rails code base where I think it makes sense
to calculate elapsed time more precisely by using
`Concurrent.monotonic_time`:

- Fix calculation of elapsed time in `ActiveSupport::Cache::MemoryStore#prune`
- Fix calculation of elapsed time in
  `ActiveRecord::ConnectionAdapters::ConnectionPool::Queue#wait_poll`
- Fix calculation of elapsed time in
  `ActiveRecord::ConnectionAdapters::ConnectionPool#attempt_to_checkout_all_existing_connections`
- Fix calculation of elapsed time in `ActiveRecord::ConnectionAdapters::Mysql2Adapter#explain`

See
https://docs.ruby-lang.org/en/2.5.0/Process.html#method-c-clock_gettime
https://blog.dnsimple.com/2018/03/elapsed-time-with-ruby-the-right-way

Related to 7c454214

The `distinct` affects (reduces) rows of the result, so it is important
part when both `distinct` and `offset` are given.

Replacing SELECT clause to `1 AS one` and removing `distinct` and
`order` is just optimization for the `exists?`, we should not apply the
optimization for that case.

Fixes #35191.

Relation is not best place to do this.

Address test_belongs_to_does_not_use_order_by failure

Address `test_belongs_to_does_not_use_order_by` failure due to checking order by for metadata queries

Also, `test_has_one_does_not_use_order_by` should not check metadata queries.

Fixes #35098

Finish rename of database_operations to database_resolver_context

Improve naming in DatabaseSelector

Fix `relation.create` to avoid leaking scope to initialization block and callbacks

`relation.create` populates scope attributes to new record by `scoping`,
it is necessary to assign the scope attributes to the record and to find
STI subclass from the scope attributes.

But the effect of `scoping` is class global, it was caused undesired
behavior that pollute all class level querying methods in initialization
block and callbacks (`after_initialize`, `before_validation`,
`before_save`, etc), which are user provided code.

To avoid the leaking scope issue, restore the original current scope
before initialization block and callbacks are invoked.

Fixes #9894.
Fixes #17577.
Closes #31526.

Don't use `false` as special value to skip to find inherited scope, we
could use `skip_inherited_scope = true`, and move `_scoping` back on
Relation.

Move compiled ERB to an AV::Base subclass

Revert "Remove finalizer and configuration"

This reverts commit 9e7b4a3173788ea43b11e74a4d2f69a5f1565daa.

Then we don't need the extra module.

Now we can throw away the subclass and the generated methods will get
GC'd too

We're already in the resolver, we call this class "operations" in the
middleware, so we should use the same naming here.

This is more consistent with Resolver, which has build called. This
allows using a Proc instead of a class, which could be nice if you need
to vary switching logic based on the request in a more ad-hoc way (ie.
check if it is an API request).

Fix `CollectionProxy#concat` to return self by alias it to `#<<`

Improve execution time of railties' test 

Formerly it was returning arguments (`records` array).