[5.9] EachById

[halaei]

Reopening #28024 (targetting 5.8) for laravel 5.9. This is also a refactor for chunkById() and actually reduces the total lines of codes (except for tests) while adding a new feature.

New Feature:
eachById() is a more efficient alternative to each(), just like chunkById() is to chunk(). The runtime complexity is not quadratic and it works fine for read-write scans as well.

Changes:

• chunkById() functions in Query\Builder and Eloquent\Builder was mostly duplicate, so I removed them and move their main parts to BuildsQueries trait.
• The different parts of chunkById() in the 2 classes are now moved to defaultKeyName() function.
• The default value for $column in Query\Builder::chunkById() was 'id'. Now it is null.
• The description of chunkById() is changed from "Chunk the results of a query by comparing numeric IDs." to "Chunk the results of a query by comparing IDs." because it handles non-numeric ids as well.