[5.1] Eloquent relations improvements

[rkgrep]

Work is still in progress, however I want to know if you would accept such changes

The patch applies two fixes to eloquent belonging relations:

1. Drop relation on change of relative field

$child = new Child();
$parent_id = $child->parent->id; //Assume parent_id is 888. Parent is still there and loaded

$child->parent_id = 999;
$child->save();

$parent_id = $child->parent->id; //We still get old parent loaded previosly - which is a bug

The patch forces such relations to be cleared on model update.

onDirty method added to provide internal saving event without Event package.

1. Inject parent to children on resolving having relations (fixes Odd Relationship Behaviours #8085)

$child = $parent->child; //HasOne, HasMany, MorphOne, etc. relation

$parent_id = $child->parent->id; //New query which can cause n+1 problem

$childrent = $parent->children()->with('parent'); // Works but nonscence - unnescessary extra query.

In this patch parent model automatically injected into children in one-to-many relations.

Next - I plan to fix #5414, #6368 and implement some more relations improvements.