Refactor cursors to inherit from BaseCursor, scroll with a Mongoid::Scroll::Base64EncodedCursor

[dblock]

@FabienChaynes take a look, this lets you scroll normally.

Feed::Item.desc(:position).limit(5).scroll(Mongoid::Scroll::Base64EncodedCursor) do |record, next_cursor|
   # next_cursor is of type Mongoid::Scroll::Base64EncodedCursor
end

I removed support for Mongoid 3, 4 and moped, I think it's time.

[FabienChaynes]

Base64EncodedCursors are not working properly when the sort field is not a String one. Cf. comment below.

[dblock]

Specs passing. There's probably a more methodical way to approach writing these tests than the lots of copy-paste.

[FabienChaynes]

BaseCursor.parse_field_type returns the class (e.g. Date) while BaseCursor.extract_field_options used in Cursor return the class as a string (e.g. "Date").

With the class, the scroll method will raise a MismatchedSortFieldsError because the class will be compared with a string in Scrollable::Fields#different_sort_fields?. We could either use to_s in BaseCursor#sort_options or not use BaseCursor.parse_field_type here to do things like in Cursor (if so, the method will have to be deleted):

Suggested change

	field_type: BaseCursor.parse_field_type(parsed['field_type'], parsed['field_name']),
	field_type: parsed['field_type'],

[3] pry(main)> c=nil; Feed::Item.asc(:name).limit(2).scroll(Mongoid::Scroll::Base64EncodedCursor) {|_, cr| c = cr}; c

[7] pry(main)> Feed::Item.asc(:name).limit(2).scroll(c).pluck(:id)
=> [BSON::ObjectId('6075ad853298c919807b4b52'), BSON::ObjectId('5e8d90cf5bd5441f2894f2e7')]

[8] pry(main)> Feed::Item.asc(:name).limit(2).scroll(Mongoid::Scroll::Base64EncodedCursor.new(c.to_s)).pluck(:id)
Mongoid::Scroll::Errors::MismatchedSortFieldsError:

[dblock]

@FabienChaynes good eyes, not sure how you saw this 😅 fixed, added specs to reuse both the cursor returned and the serialized cursor for all types

[FabienChaynes]

TBH, I wouldn't have noticed it without a manual test 🙂

LGTM, just one question about a new dependency that I don't understand.

[FabienChaynes]

Why is it needed?

[dblock]

I ran into https://stackoverflow.com/questions/60226893/rails-nomethoderror-undefined-method-new-for-bigdecimalclass with a newer ruby version. We don't use that in CI, maybe I'll add it.

[dblock]

Thanks for a thorough code review!