[5.9] Use eager loading optimization for all integer keys

[rimace]

With version 5.7.14 there where some performance optimizations for the whereIn method for relations (noted as "Improved eager loading performance" in the release notes).
This brought a breakage for users that didn't declare the primary key type in their models. So the optimizations where limited to primary keys which are auto-incremented and declared as integer keys.

As it is OK to break things for a new version I propose to drop the auto-increment requirement and make use of the performance optimization for all integer primary keys.

Side note: at the time the $keyTypevariable was introduces it was not neccessary to set it, as it was not really used, which was probably the reason why it was not noticed in the upgrade guide. As this is a breaking change (for those who still don't know about this variable) this change should be noted in the upgrade guide (see my comment in issue 26582 ).

Also see Idea 1561

[taylorotwell]

What is the breaking change? Provide a detailed upgrade guide required for this change.

[rimace]

Users need to declare the correct primary key type in the $keyType variable in their models. This is the only thing that needs to be mentioned.
Long time Laravel users don`t know about it as it wasn't mentioned in any upgrade guide and they don*t read the whole documentation about models again (which is why it broke applications when this change was introduced in 5.7.14).

Example: If the primary key is a string but the $keyType variable was not changed (set as int as default), applications may break when a model eagerly loaded.