Fix the leaking scope of `each_batch`

What does this MR do?

This MR fixes the leaking scope problem of the each_batch method.

Since we are extending the class interface of our models with EachBatch::each_batch, calling each_batch on an instance of ActiveRecord::Relation ends up being handled by the ActiveRecord::Delegation::ClassSpecificRelation#method_missing, and this method preserves the current scope to allow chaining multiple scopes and class methods. Because of this, all the queries generated within the context of the block in each_batch are applied on top of the previous scope.

User.where(sign_in_count > 100).each_batch do |batch|
  ...
  do_something_unrelated(...)
  ...
end

def do_something_unrelated(...)
  User.where(id: 1) # applies both sign_in_count > 100 and id = 1 filters.
end

Possible solutions to this problem

1. Move EachBatch to a non-autoloadable directory and include it to ActiveRecord::Relation
2. Instead of including EachBatch to model, include it to Model.relation_delegate_class(ActiveRecord::Relation)(Danger: The relation_delegate_class method is part of the private API)
3. Clear current_scope before yielding the relation to user given block

This MR fixes the problem by choosing the solution 3 from the above list.

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• [-] I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team